Glenda,
It depends on how up to date your NT4 install is and what version your master server is.
I believe the 5.x client requires a certain SP level (SP6a I think) of NT4, which, of course, requires a certain level of Internet Explorer.
If you're already up to date on all that, then you should be OK with the 5.x client.
Of course, if your master server is 6.0, you'll probably want the 5.x client.
If your master is 5.x, you're also OK with the 4.5 client.
That's all in terms 'official support', of course.