Mach 4 is more Fanuc compatible than Mach 3. Patterned after a 21i. Cutter comp memory type C. For regular XYZ programs, the Mach 3 post is probably adequate. If not, start looking through the Fanuc posts.
Lathe/Turn is where the more major differences from Mach 3 are. Specifically G76 threading. Mach 4 accepts both Fanuc G76 1 line and two line formats.
Steve