It would take all
4 yonko and their crews + fleets combined in order to
match the strength of the marines back in marineford
now go cry, illiterate
If all 4 yonkos combined, they will kick Marine's ass even if shichibukai are with them.
The fact that every yonko are not on friendly term w/ each other makes "balance" or maybe we better use "stalemate" position.
I say maybe 2 yonkos can take on Marineford without shichibukai..
If shichibukai are helping, then maybe it would takes 3 yonkos to wipe Marines & shichibukai
This is proven when shanks arrives, sengoku knew Marines can't take more than that (even after they defeat whitebeard)..
But Marines is just 1 branch of the world government.. all yonkos combined won't be enough to takes on whole WG in open war.