Mach3 can handle programs up to 10,000,000 lines, so unless you need bigger than that, it's not a reason to upgrade. But most people do need to turn off the toolpath display in Mach3 with very large files, as the display uses a lot of resources.
Mach4 is a completely different program from Mach3, written from the ground up. WIll it make your machine faster? Maybe, maybe not. Most users claim that the motion is indeed better than Mach3, mainly because Mach3's trajectory planner has several bugs, and limitations (It was written over 10 years ago).
The extremely long delays in Mach4 becoming a viable product led many people to look for alternatives. And there's also the issue of Mach4 relying on third party hardware and plugins. If there's an update to Mach4, or new features get added, you have to wait for the hardware manufacturer to support them. This can take months, or even longer. Warp 9 has been working on their Mach4 plugin for their ESS for at least 4 years? And it still does not support all of Mach4's features.
Most people using Mach4 seem to be very happy with it, though.
I decided to switch to UCCNC, which is very similar to Mach3. It's actively developed, and quickly gaining in popularity. And for the cost of a Mach4 license, you get the software and hardware.
It's not perfect, but it's better than Mach3, and getting better all the time.
Other will tell you that the Centroid Acorn is another good option. I've heard it provides the best motion of any of the low cost controls.
Gerry - http://www.thecncwoodworker.com