Can I use my SunOS 4 disks on Solaris 2.x?

Yes you can. The on disk format in Solaris 2.x isn't
different from SunOS 4.1.x, as long as they've been formatted
under SunOS 4.1.x. Disks formatted on older fses need to be converted
with "fsck -c". If "dumpfs | head" (SunOS 4) or "fstyp -v | head"
(Solaris 2) lists "format dynamic" as one of the first lines, the
disk does not need to be converted.

UIDs > 60002 may give problems when moving disks from SunOS 4.x
to Solaris 2.x. This is fixed in a Solaris 2.5.1 which has
MAXUID defined as 2147483648. Note too that UID and GID 60001
and 60002 have been defined as nobody and noaccess on Solaris 2.x.
If the target SunOS 4 system uses such IDs, you need to renumber
them to avoid the collision

Moving disks the other way around may give problems: Solaris 2.5
supports on disks ACLs, and when MAXUID in 2.5.1 is increased
beyond 65535, that will give added difficulties.

ASGAUR