You misunderstand my post - there are only ever four powers, with the neutrals acting just like OOB, but with different alignments.
It really should be:
1939- June 1941 Four Power game (Allies[UK & F], Axis[G & I], USSR, Japan)
Becomes three power game after June 1941 (i.e. USSR joins the Allies)
Becomes two power game after December 1941 (Japan joins the Axis)
It’s reasonable to assume that the USA only ever joins the Allies, and that (before June 1940) Italy only ever joins the Axis. The position of China is tricky, since we don’t want to exclude the possibility of an Allied/Japanese alliance, but where does that leave China - a neutral that the other powers (i.e. not Japan) can send aid to without it being a DOW on Japan?