1. 3V = 3 valves per cylinder, 4V = 4 valves per cylinder. In general, more valves means more intake and/or exhaust... more power.
2. 03/04 Mach vs. 05/07 GT: 4V Mach has just a bit more HP, arguably a bit faster, old design, old school, lots of easy mods, lots of fun, missing my 03. Test...