I'd say the chance of a free upgrade would be very small.
When I was creating my 2010 screenset in October of 2010, I was told to wait because V4 would be out soon. The latest estimate is still probably 6 months until release. I'd guess over a year until the bugs are worked out.
If you need the licensed version, then buy Mach3 now. Noone is going to force you to switch to Mach4, so you can keep using Mach3 as long as you like, and switch over if you find that you need some features that only Mach4 has.
Also, be aware that mach4 is going to come in different levels, at different price points.