'R4' was a British unrealised contingency plan for the seizure of the port cities of Stavanger, Bergen, Trondheim and Narvik along Norway’s west coast should the Germans react strongly to the implementation of 'Wilfred' (early April 1940).

A British and French staff paper of April 1939 on broad strategic policy had recognised that in the first phase of any war with Germany, economic warfare would be the only effective offensive weapon available to what would by then have become the Allied powers. In the light of this and the Allies' experience in blockading Germany during World War I, Norway inevitably assumed great significance for the Allies on the outbreak of war in September 1939. Before the middle of September 1939 the British government had made its first attempt to secure from Norway a 'sympathetic' interpretation of its rights as a neutral state, but Winston Churchill, the First Lord of the Admiralty, was already engaged in devising more active measures. On 12 September he submitted his 'Catherine' plan for sending naval forces through the straits leading into the Baltic Sea to gain control of those waters and bring too an end the maritime shipment of Swedish iron ore to Germany, but as this involved extensive alteration of several battleships to give them greater protection against air attack, it could not be put into effect at an early date. At the end of the month Churchill suggested mining Norwegian territorial waters to cut the ore route from Narvik, the ice-free northern Norwegian port at the north-western end of the railway line from Sweden’s iron ore-mining region in the area of Kiruna. In December Churchill renewed his efforts to win approval for the mining of the Leads, but could not obtain a decision.

During the early months of the war there was a strong tendency between the two Allies to base hopes on the Germany’s supposed weakness in terms of strategic natural resources, and thus the Norwegian port and the Swedish ore began to loom very large in their thinking. Late in November the British Ministry of Economic Warfare expressed the view that, cut off from the Swedish ore supply, Germany could not continue the war for more than 12 months and, deprived of the supply which passed through Narvik, would suffer 'acute industrial embarrassment'.

On the other hand, Grossadmiral Erich Raeder, the commander-in-chief of the German navy, believed that Germany could stand the loss of anything between 2 and 3.5 million tons of ore per year delivered via Narvik and that, by storing ore in Sweden during the winter for summer shipment through the Baltic after the winter ice had melted, could probably reduce the annual loss to about 1 million tons.

Subsequent Allied planning was centred on the decisive significance of the Swedish ore, often to the extent of failing to recognise the many difficulties of securing and holding both Narvik and the ming region of Kiruna and Gällivare against the determined German counter-action such a move would undoubtedly trigger.

At the end of November the Soviet 'Talvisota' winter war on Finland created new possibilities for the Allies by arousing a hope that the Scandinavian countries, out of sympathy with Finland and on the ground of their obligations as members of the League of Nations, might permit Allied troops to be sent to the aid of the Finns across their territory. Such an undertaking could be made to include the occupation of Narvik and the area of Kiruna and Gällivare as a by-product as the railway linking Narvik on the coast of the Norwegian Sea and Luleå on the coast of the Baltic Sea provided the most direct route to Finland. The French government went as far as to consider the establishment of a major theatre of war in Scandinavia as a means of drawing the main focus of imminent military action away from the Franco-German frontier. On 19 December, however, when the French prime minister Edouard Daladier proposed the dispatch of an expeditionary force to Finland, he met opposition from the British, who were fearful of provoking a breach with the USSR.

When the early successes of the Finns made it appear that the Soviet army would be a weak adversary, French enthusiasm for a second front in Scandinavia grew. After Sotamarsalkka Carl G. E. Mannerheim, the commander-in-chief of the Finnish forces, on 29 January appealed for support, the Allied Supreme War Council decided to send an expedition in the middle of March 1940. The French wished to blockade Murmansk and attempt landings in the Pechenga region, and talked in terms of simultaneous operations in the Caucasus in addition to the occupation of parts of Norway and Sweden. The British plan, which was adopted, was more modest and, while ostensibly intended to bring Allied troops to the Finnish front, laid its main emphasis on operations in northern Norway and Sweden. The Allied main force was to land at Narvik and advance along the railway to its eastern terminus at Luleå, occupying Kiruna and Gällivare in the process. By a time late in April, two Allied brigades were to be established along that line, and a third Allied brigade was then to be forwarded to Finland. A secondary force of five British Territorial Army battalions was to occupy Trondheim, Bergen and Stavanger to provide defensive bases in southern Norway. Stavanger was to be held only long enough to allow the destruction of its airfield, while Trondheim was to become the major base in the south and the port of debarkation for Allied troops sent into southern and central Sweden to meet the expected German counterattack. Eventually the British and French intended to commit to this theatre as many as 100,000 and 50,000 men respectively. The Allied effort moved slowly, and massive Soviet offensives in February rapidly wore down the Finnish resistance, and the execution of the Allied plan meanwhile remained contingent on the willingness of the Norwegian and Swedish governments to grant rights of transit to the Allied troops. A Finnish request to that effect was denied on 27 February, and another by the British and French governments on 3 March. By that time the Finns had decided to open peace negotiations with the Soviets. On 9 March the Finnish ambassadors in Paris and London were informed that, should the Finns issued a call for help, the Allies would come to their aid with all possible speed. The Allies promised delivery of 100 bombers within two weeks, but the dispatch of troops still remained dependent on the attitude of Sweden and Norway. On the same day Mannerheim, who regarded the Allied proposal as too uncertain, gave his government categorical advice to conclude peace.

At the last minute, on 12 March and still hoping for an appeal from the Finns, the Allies decided, at the suggestion of the French, to attempt a semi-peaceable invasion of Scandinavia. Assuming that the recent diplomatic responses of the Norwegian and Swedish governments ran counter to public opinion in those countries, they proposed to 'test on the Norwegian beaches the firmness of the opposition'. A landing was to be made at Narvik and, should this succeed, to be followed by one at Trondheim while the forces for operations at Bergen and Stavanger were to be held in readiness. The objectives were to take Narvik, the Malmbanan (iron ore line), and the Swedish ore region, but the landing and the advance into Norway and Sweden were to take place only if they could be accomplished without serious fighting. The troops were not to fight their way through either Norway or Sweden and were not to use force except 'as an ultimate measure of self-defence'. The treaty which Finland signed in Moscow on the night of 12/13 March then brought to an end to the the Allied hopes, and the forces which had been assembled in the UK were released for other tasks.

On 21 March Paul Reynaud became the head of a French government that was now committed to a more aggressive prosecution of the war and, one week later at a meeting of the Supreme War Council, the Scandinavian question was once more considered. The new Allied undertaking was to consist of two separate but related operations, 'Wilfred' and 'R4'. 'Wilfred' involved the laying of a pair of minefields in Norwegian waters, one in the approaches to the Vestfjord to the north of Bodø and the other between Ålesund and Bergen, with the pretended laying of a third near Molde. The laying of the minefields was to be justified in notes delivered to Norway and Sweden several days in advance protesting those nations’ inability to protect their neutrality. The supposition was that 'Wilfred' would provoke a German reaction, and 'R4' was to become effective the moment the Germans landed in Norway 'or showed they intended to do so'.

The 'R4' plan called for a response immediately after any German violation of Norwegian territorial waters, and to ensure speed of response the heavy cruisers Berwick, Devonshire and York and the light cruiser Glasgow embarked troops at Rosyth on 7 April, while the men and transports for the Trondheim and Narvik landings were assembled in the Clyde river together with the light cruiser Aurora and six destroyers, all under the command of Vice Admiral Sir Edward Evans.

Narvik and the railway to the Swedish border were the principal objectives. The port was to be occupied by one infantry brigade and one anti-aircraft battery, and the total strength to be developed to an eventual total of 18,000 men. One battalion, in a transport escorted by two cruisers, was to sail within a few hours after the mines had been laid. Five battalions were to be employed in occupying Trondheim and Bergen and in raiding Stavanger to destroy the airfield at Sola. The battalions at Trondheim and Bergen would later be reinforced by the troops from Stavanger if the movement could be managed, but otherwise they were to rely on their own resources. The success of the plan depended heavily on the assumption that the Norwegians would not offer resistance and, somewhat oddly, the possibility of a strong German reaction received almost no consideration.

The execution of 'Wilfred' and 'R4' was initially linked to 'Royal Marine', the British plan to sow mines in the Rhine river, to which the French objected on the ground that it would provoke German bombing of French factories. 'Wilfred' had been scheduled for 5 April, but it was not until that date that the British government agreed to carry out the Norwegian operations independently of 'Royal Marine'. As a result, the mines were not laid until the morning of 8 April, at which time the German ships for the 'Weserübung' invasion of Norway were already passing to the north along the Norwegian coast. When it became known on the morning of 8 April that the German fleet, which aircraft had sighted on the previous day, was at sea in the vicinity of Norway, the minelaying force was withdrawn, 'R4' abandoned and the British fleet was ordered to sea in an attempt to intercept the German naval force.