Haven't tried this yet buy got it from wolverine15 on ps3hax.net.
How to fix MGS4 act 4 (PSX Part)
This is a tutorial on how to fix Act 4 in Metal Gear Solid 4. This is the part where it switches to the PSX part at shadow moses. (At time of writing) Without this fix it will crash. This is caused because the ps3 switches to PSX emulated mode and when it tries to switch back to regular ps3 mode it locks up and freezes.
The original tutorial can be found here. I have made some alterations to make it easer to understand.
Step 1: Start PS3 in Jailbroken State (unplug/turn of switch at back insert device plug in/turn on switch press power then eject in quick secuence.)
Step 2: If you have played MGS4 already on your console delete the GAME data. do this by going to the game tab and going to the very top where it says game data utility then find your MGS4 data press triangle and delete. MAKE SURE THIS IS YOUR GAME DATA NOT YOUR SAVE FILE.
Step 3: Install and start Gaia manager (must be higher then ver. 1.03.1) highlight your MGS game on your hard drive and select patched mode with L1 this will fix ACT 4 and let you finish the game without the retail disc in.
Step 4: Start the game and install the data again.
after completing these steps you will be able to boot MGS4 with any backup manager as your game data will be patched.
Read more:
http://www.ps3hax.net/showthread.php?t=16553#ixzz1D89rCPGE