If you can make it to the Early Startup screen, you probably don't have a hardware problem. And if a 4000T can't find workbench.library, it'll open an empty screen called Workbench Screen and then stay there (because LoadWB will have failed and the startup-sequence will have ended).
So it sounds like something is failing in the startup-sequence or user-startup if all you're getting is a black screen. Boot with no startup-sequence, then type
set echo on
execute s:startup-sequence
to see where it's choking.
