'Plan 4' was the US plan for the defence of the western hemisphere after the 'Greer incident' (September/11 December 1941).

At 08.40 on 4 September 1941 the US destroyer Greer, carrying mail and passengers to the US garrison in Iceland, was signalled by a British aeroplane that a U-boat had crash-dived some 10 miles (16 km) ahead of her. About 40 minutes later the destroyer’s sonar operator detected U-652, which Greer began to trail. The British aeroplane, running short of fuel, dropped four depth charges at 10.32 and returned to base, while Greer continued to follow the U-boat. Two hours later the U-boat began a series of radical manoeuvres and Greer's look-outs saw the boat pass at a distance of about 100 yards (90 m). At 12.48 Greer detected a torpedo launch, and her captain ordered flank speed and a hard turn to port. The look-outs watched the torpedo pass 100 yards (90 m) astern and the warship then attacked, laying a pattern of eight depth charges which missed. Less than two minutes later a second torpedo passed 300 yards (275 m) to port.

Greer lost sound contact during these manoeuvres, and began to quarter the area in search of the U-boat. After two hours the destroyer re-established sound contact and laid down a pattern of 11 depth charges before discontinuing the engagement. Greer had held the German boat in sound contact for 3 hours and 28 minutes, had evaded two torpedoes fired at her, and with her 19 depth charges had become the first US warship in World War II to attack a German naval target.

When news of the attack on the US warship reached the USA, public feeling ran very high, and President Franklin D. Roosevelt seized the opportunity of growing movement from isolationism toward an anti-German sentiment to deliver one of his celebrated 'fireside chats', in this instance bringing the USA closer to outright involvement in the European war.

Declaring that Germany had been guilty of an act of piracy, Roosevelt in effect unleashed US warships and aircraft for offensive action in, as he put it, 'water which we deem necessary for our defence' and went on to day that 'American naval vessels and American planes will no longer wait until Axis submarines lurking under the water, or Axis raiders on the surface of the sea, strike their deadly blow – first.' With this 'shoot on sight order', there began the period of 'undeclared war' between the USA and Germany in the North Atlantic. This lasted until Germany declared war on the USA on 11 December 1941.