NT4 (NT standing for "New Technology) is / was an OS designed for business and professional markets, as opposed to Win95/98/Mill which were designed for home enviornments. It has been replaced by Windows 2000.
The development cycles for the NT family:
NT 3.5
NT 4
Win 2000
Win XP (later this year)
vs.
DOS
Win 3.1
Win 95
Win 98
Win Millenium
Win XP (later this year)
While the the Win 95 "family" has a core based on good 'ol DOS, for compatibility with old stuff, the NT "family" bypasses this. The result of these two seperate developments are:
NT is more stable, less prone to crashing.
NT being for "professionals" is less "user friendly"
NT has fewer software drivers for "consumer" products.
The NT family has traditionally had poorer support for "multimedia" because of Microsoft's perception that this is more of interest to home users (i.e. game players). But as you can see the systems are converging. The interface in Win 2000 and Win Millenium are almost identical, XP should finally bring the familys together (and drop the last remnant of DOS support).
If your hardware has drivers for NT, or Win 2000, and you are going to use the system exclusively for music, I would say that is the best choice.
In my case, as I am very familiar with Win 98, and I do occasionally use my DAW system for other things, I use that system. It works fine for me.
YMMV.