I don’t think it matters all that much in a 4 man, it can work either way, with certain benefits and drawbacks to each pairing. Its kind of amusing on occasion to let the Allies choose whether they want to play the British or the Americans first, and then, if they can’t decide who will take the lead, flip a coin to see who gets to be “Supreme Allied commander!” (ie. take control of Russia.)
:-D
I agree that the UK/Russia combo is probably faster, and gives the Allies a stronger edge coordinating at the Center, Karelia/India/Caucasus etc etc. but then the USA can also be rather boring to play solo. A lot of waiting, with little action in early rounds.
In Classic and Revised, it was sometimes fun to encourage an Allied split with Churchill on one side and FDR/Stalin on the other (kind of mirroring the historical situation/mistrust between the leaders of the British and the Russians). In a Face to Face game, this could sometimes compensate for the Axis starting disadvantage on those boards OOB, just due to the difficulty of coordinating as Allies. This also worked better in PBEM play, with just one turn in the email exchange for USA/Russia.
But in AA50 and 1942.2 where the advantage is to Axis, I think it might be better to just pair Russia with UK, as others have suggested if your main concern is balance by sides among players of roughly equal skill. British fighters or tanks working with Russian stacks at the center, saves on the amount of time that the Allies will spend bantering and pleading with each other. USA starts way out, and usually their coordination with the Royal Navy Atlantic, or in a KJF, is pretty straightforward once the game gets to that point.
I like the Russian bomber for a number of reasons. But if you feel that the game still tilts towards Axis too heavily, you might enjoy giving any additional bid to USA, rather than UK. A bid of 5 ipcs to USA can be fun. This prevents a naval build, and instead puts the focus in China. This can be good for Allied center control, and alleviate some of the pressure on Allies, in a way that they wouldn’t usually adopt otherwise. At 6 ipcs or more, the USA can really disrupt Germany’s best u-boat option and make an Atlantic crossing much faster for the Allies. That has merit too, if the Allies really need a leg up, but I like sticking them with a 5 spot. This allows for a single extra infantry or artillery unit in China, with a remainder of either 2 or 1 ipc left over for more variety at USA purchase. Having one extra unit in Szech actually does a lot to free up the Russians, and it gives USA more power at the center, if that 3rd ground unit in Szech can be kept alive until American fighters/bombers start arriving mid-game.
The Russian bomber alone is pretty potent though. I think it probably makes the Russians more enjoyable to play in either pairing, but especially if your players decide they want to pair USA with Russia, rather than UK with Russia.