I know every situation is different, so here is my Opinion on it.

mach has a HUGE following.  I have used it since 2001-2002.... it was different product then. 
mach3 has been my go to since 2006.  There is a huge costumer base and a huge forum base for help.
Mach3 license can also be transferred from one computer to another in case of failure.  I think I read that mach4 is a single computer license.
My opinion is for your first machine adventure.  Go with what you know will work with the least amount of effort.  More people can help with mach3 then with mach4 at this time.
Get more knowledge under your belt, then in the future you can always move to Mach4, or if your happy and it works, stay where you are.