Enormous GTA 5 mod adds GTA 4's Liberty City as a fully functional map, with a working subway, traffic, and even the notorious swing set glitch
It's still a work in progress, but the mod already works seamlessly in the regular ol' single-player version of GTA 5, as ...
Dexerto5d
You can go to Liberty City in GTA 5 with this mod of the entire GTA 4 map
Modders have brought the entirety of Liberty City, as it was in Grand Theft Auto 4, to GTA 5, and it’s easy to switch between back to Los Santos at any time, too.