It looks like the end of the line for FreeBSD 4.x, now that the 5.x branch has stabilized with 5.3. Upgrading to 5.x on production systems has not been recommended before now, but the procedure seems pretty severe all the same.
FreeBSD 5.3-RELEASE Migration Guide:
Probably the most straightforward approach is that of “backup everything, reformat, reinstall, and restore user data”. This eliminates problems of incompatible or obsolete executables and configuration files polluting the new system. It allows new file systems to be created to take advantage of new functionality (most notably, the UFS2 defaults).
Sensible, though. The lazy man’s way would be to install on a new disk in a spare chassis (I have an old Pentium II that would work) and then just swap it in to minimize downtime and add storage into the bargain. Time to sniff around on eBay and see what’s available.