I've got DIGITAL Unix 4.0E running on mine (because it's the last version to be called DIGITAL Unix), otherwise I would recommend upgrading to 4.0G or 5.1B (although 5.x doesn't feel as speedy as 4.0, even on my XP1000).
I personally like Digital Unix, besides standard stuff like acrobat reader and netscape, there is one particular piece of software, that stands out, and this is Symbolics OpenGenera, a runtime environment for Symbolics' LISP-Machine OS on DIGITAL Unix (it does only work on 4.0 out of the box).
I don't know too much about NT4 on AXP, as my XP1000 is the faster version with EV67 and this one doesn't support NT anymore
EDIT: If you're feeling adventurous, there's always OpenVMS
you can get VMS licenses from openvmshobbyist
Licenses for DEC OSes are not bound to a specific version, I'm running my DU 4.0E with a Tru64 5.1B-4 license.