No, they weren't necessary. The use of 6 engines was primarily a way of using the weaker 1450 hp Jumo 211J or P engines and still get a good take off run. Even with 4 Jumo 211P engines (1500hp ea) the Me 264 could do the mission, it just needed an unrealistically long runway: 2500m or so, hence it would need to use RATO which was unattractive. There wasn't enough hanger space for the Me 264, Milch wasn't disposed in a kindly manner to Messerschmitt, he felt it an extravagance, so its development wasn't supplied with enough engineers, hanger space, draughtsmen. A small token 'Doolittle' style on the US east coast seems possible in late 1944 had it of been pushed. The Me 364 (ie Me 264/6m) of course would have superior performance than the 4 engine versions given the same engines.