I'm running into some limits of SunOS 4.x, will upgrading to

Solaris 2.x help?

The answer depends on the limit you run into.

Solaris 2.x supports filesystems upto 16TB, SunOS 4.x requires
ODS 1.0 to support filesystems over 2GB.
Solaris 2.6 and later support files > 2GB.
Solaris 9 08/03 supports ufs filesystems of upto 16TB on 64 bit
SPARC kernels.

Swap partitions and files are still limited to 2GB a piece as long
as you run a 32 bit kernel, but you can have multiple 2GB swap
partitions/files.

Solaris 2.x supports a virtually unlimited number of open
filedescriptors, SunOS 4.x only supports 256 (default) or 1024
(with Sun DBE 1.x).

Solaris 2.x supports an unlimited number of pseudo terminals.
SunOS 4.x supports at most 256.

Solaris 2.x supports more SCSI disks.

Solaris 2.x limits can be tuned in /etc/system, requiring just a
reboot. SunOS 4.x limits need to be tweaked in the config file and
a new kernel needs to be built and installed.

Solaris 7 and later in 64 bit mode support > 4GB of address space
per processes.

64 bit processes in Solaris 7 and later can open more than 256 files
using stdio.

Solaris 9 LVM supports logical slices, allowing more than 7 usable
slices/partitions per disk.

NOTE: when the above says "unlimited", it just means that there is
no "hard" limit, but performance may degrade over certain values.
E.g., setting the number of available fds very high, will cause
programs that loop closing all fds to be very slow in starting.

ASGAUR