That depends - on you, your situation, your application mix, etc. Some year SunOS4.1.x will go the way of the 3/50 - it'll still be around, but Sun will no longer support it.
You don't have to upgrade immediately, but you should be planning your upgrade path by now.
If you would rather stay with a BSD-style operating system, you should consider OpenBSD or NetBSD. OpenBSD is regarded as the world's most secure OS, and NetBSD is the world's most portable OS. Both are BSD-style OSes in the spirit of 4.xBSD, which SunOS4 was also derived from. Both run on SPARC, have SunOS emulation, and are Y2K compliant.