If you can still find it, I'd upgrade to SP3 first, and make sure all is well before taking the plunge to SP4. Personally, I don't think SP4 offers as much "patching" as SP3, and from what I've read, SP4 includes patches to all services for Windows 2000 (such as ISA, and other services you may not be running).. I've personally seen a significant amount of issues upgrading to SP4, and seen that SP3 works much more reliably.
But, if you really need to get to SP4, I'd see about cleaning up as much space on your C drive as possible. Sometimes this means uninstalling some apps and moving them to another drive, but you should be able to clean out your TEMP folders, and the old logfiles in \winnt\system32\logfiles\.
Hope this helps.
Mike C.