Windows 2000 License

Shortly after accepting a new position, I immediately did a software audit. I found us to be short Windows NT 4.0 WS license. My question is, rather than purchase Copies of NT4.0 can I purchase the full version of Windows 2000 Professional, and still use NT4.0 on the workstations. The machines will eventually be changed out within 2 years at that time Windows 2000 will be installed. In the mean time I would rather keep NT4.0 WS on these computers.