I would say 6 but it's not on there Martok could you ad it?

Reason why is 12 would be to much altough it will give a bigger impact in the game, they could even make it so that certain months would have heavier snow or colder etc.

4 would give you just the seasons, and is defintly better then 2 . But then you could say Oh it's december I'wont invade Russia because it's winter a bit to predicatable I think.

That's why I'ld say 6, you could have a winter peak already November, December but less in Jan and Feb. Just to give a example. Or droughts in certain summer months that would give bad crops or shortage of water for troops, and people. and just troop movement will be even more realistic.
It will be less of on impact for a to long gameplay then 12.

But I will be already a realy happy camper with 4