In MGS4 the player has no limit on the amount of saves they can make during a play through, however, over the course of the game, the PS3 system only supports the creation of a limited amount of saves [100], which would be hit just before act 5 if the player was saving at every 'checkpoint'.
Players may create their own save data or use others save data with some restrictions. When copying another users save data, file creation version, date, time, Drebin points, in game items & equipment, and in game time are all transferred across, however, the ability to earn trophies is disabled when using save data created by another account. In addition 1.0 is not compatible with 2.0 data and will result in an error showing corrupt data, though it is simply not compatible. 2.0 can read 1.0 data perfectly fine.
In order to use another users save data on your system, a simple list of instructions needs to be followed. You will need a few items:
1) Save data is named based on the version of the game you are playing. Find which save data you need for your version, usually BLUS30109 for NTSC-U, and make sure the folder you have is named that, plus a seemingly random string of Hex Code, e.g. BLUS30109G5D6B12BE; do not rename it or the data may break. Usually this folder will be inside of a folder named 'SAVEDATA' inside of a folder named 'PS3', if not, make sure it is.
2) Plug a FAT32 formatted removable storage device into your computer and drag and drop the 'PS3' folder onto it. Safely remove the storage device from your computer.
3) Plug the removable storage device into your PS3 system and find 'save data utility (PS3)' under the 'Game' tab. Click it, find the storage device at the top, click it, find the save data according to its data, and press triangle on it. Then select copy to copy the data to your system.
When copying save data from your system for others, simply perform the steps backwards. Be sure to note your save datas date and version. The PS3 automatically creates the required folders but you may need to look up the name of the save data.
Check the following page for the currently uploaded MGS4 Saves Package.
More files can be uploaded as requested.