Technically, either TerraFarm version will work on both 2x and 4x maps. If you use the standard TerraFarm, and open a 4x map, it will come up with an error saying the density of the map isn't supported and TerraFarm may not work as intended. You can generally just ignore that, and there really aren't any issues.
However, using a TerraFarm meant for a 4x map, one, you don't get that error message anymore. And two, it will interact with the map in a manner closer to how the landscape tools work. It just makes it a bit more smoother.