I use V-Carve Pro and it has the option to create "Mach 2/3 .txt" files for G-code.  But it doesn't have the option for Mach 4 .txt files.

My question is this: If I switch to Mach 4 should those "Mach 2/3.txt" generated G-codes still run on my machine that now has Mach 4 operating it instead of Mach 3?

Yes.

The Mach 2/3 .txt files will run on Mach 4.

Tweakie.