There is no clean path here. Plus NT 4.0 has an unpatched vulnerability that made us "pull its plug" at office, home and lab.
In short, YOU CAN GET a Windows 2000 CD from Microsoft, eBay or elsewhere. Maybe XP is in your future.
As to installing NT 4.0, you boot that CD and do what you did before.
But it will be very insecure.
I am running Windows 2000 as an upgrade from Windows NT that I received with my computer about three years ago. I have lost my Windows 2000 software disc and the computer manufacturer (Compaq) says it is no longer available. Would anyone have a recommendation about how to clean my drive and start fresh without the disc? (I still have the NT disc).