I agree that China/ANZAC seems to work best as the 4th player.
I will point out the obvious: players should be planning their next moves during the other players’ turns. (and refining them as the situation changes. i.e. the Jap player moves a DD during non-combat to block your planned attack.)
When it comes to purchases everyone should be ready to IMMEDIATELY plop down the new units as soon as their turn begins. Very rarely should someone have to think what to buy in a 4 player game.
The disadvantage of more than 2 players is lack of coordination between the allies. The knock on the original game is that it was a 2 player game masquerading as a 5 player game.