if you have a ps3 with a jailbreak then you will have this problem .. when you reach act 4 the game will exit to the xmb ..
many people said that you will have this problem even if you play with original disk ..
and even without running jailbreak !.
i don't know .. for me i borrowed the game disc from a friend and i copy it on my HDD when i had this problem .. i tried like 5 guide and it didn't work so this is what i did ..

i deleted the system files for the game ( 4gb )

i played a game with open Manager in patch mode ( any game works in patch mode and doesn't stuck in loading screen . games like infamous or FiFa 2010 and not heavy ruin )

exit the game and then run open Manage and run MGS 4 in patch mode .( you can run patch mode by pressing L1 )

wait for the game data to be installed .. then load the game ..

now you can play Act 4 with out the crash :) .. have fun