First of all, there won't be a Mach4. The next version will be called Quantum. There is a Quantum test available for download, but it's basically just Mach3 with 1 added feature - S-curve accel and decel.
USB. Mach3 can currently run a USB ncPod (www.oemtech.com). In a few months, you should see a USB device called a Smoothstepper.
There has been a lot of talk on the Yahoo group recently about external devices. Art has said that there are limitations with pretty much any external device, but other than speed and IO, the parallel port doesn't suffer from the same limitations as external devices. Basically, the parallel port just plain works better. Especially since Mach3 has been sped up to 100Khz through the parallel port.
You won't see any version of Mach3 on any OS other than Windows. It's taken Art 4 years of 16 hour days (or longer) to get Mach3 to where it is today. And you'd want him to start over from scratch? He also has to support 10,000 registered users. There aren't enough hours in the day.
If you do a clean install of XP, install Mach3, and hook up your machine, you should never have to worry about "fixing it", or being spied on. It's a machine control, if your that worried, it shouldn't even have an internet connection. The DOS box you'd like wouldn't have one. XP Pro is a rock solid OS. And keep in mind that the average lay person that is not computer savvy makes up a large portion of the Mach user community. Switch it to a different OS, and the support would take 110% of Art's time.