'Ost' (iv) was a German operational study by Oberstleutnant Bernhard von Lossberg for the German invasion of the USSR and based on 'Ost' (iii) and later designated as 'Fritz' (i) (15 September/5 December 1940).

The plan was developed under the overall supervision of General Alfred Jodl, the chief of the operations staff in the Oberkommando des Heeres.

The plan’s most important effect on the final 'Barbarossa' undertaking was the adoption of a north-easterly rather than a south-easterly primary axis of advance, the latter having initially been preferred by Adolf Hitler as it emphasised the seizure of the food-producing areas of Ukraine, the industrial facilities and coal of the Don basin, and the oilfields of the Caucasus region. The key element of the plan including the concept that 'for the conduct of operations against the USSR it is first necessary to decide whether the axis of the main stroke will be directed to the north or south of the Pripyet marshes. The advantages of aiming the main stroke northward are as follows: firstly the superior possibilities for concentration of forces as a result of the better railway system, secondly the advisability of inflicting a prompt defeat on the Soviets in Baltic states, thirdly the relatively better road conditions in the operational direction, fourthly the possibility of co-operation with Gruppe XXI advancing from Finland, and fifthly the chance to reach and take Leningrad and Moscow. The advantages of aiming the main stroke northward are as follows: firstly the relief of the part of Romania threatened by the USSR, secondly possibility of supplying the German motorised formations from the Romanian and Galician oilfields (however, there are much worse communications on the Soviet side of the frontier), and thirdly the significance of Ukraine. The suggestion is that the main stroke be directed along the north axis.'

Similar arguments for north-easterly primary axis were made on 26 November in German general staff analysis.