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