The 4GB Patch is used mostly for games in the last two decades to be able to use up to 4 gigs of RAM (but typically less).
Basically, older games were released before having tons of computer memory (RAM) and 64 bit computers that would allow that extra RAM to be utilized were mainstream. Most applications in the last two decades were released as 32 bit and were typically built to access less than 1 gigabyte of RAM. This was fine at the time, but nowadays with people having more applications and such open while they play in particular, these older games can often run out of memory a lot faster than the original constraints they were built around.
For most games, the 4GB Patch gets around this by setting a game's main exe file (that launches the game) to be Large Address Aware, which effectively means it can now access up to 4 gigs of RAM and is less prone to crashing the game from running out of memory.
TS3 fully patched however does not need it. It has been Large Address Aware since the release of Late Night back in 2010. And if your game is fully patched, the game's exe would also already have this change. So you don't need to apply it to your game at all. It wouldn't hurt anything, it's just entirely useless because it's already been done by the dev team years ago.
Hope that answers your question!
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