The answer depends on the limit you run into.
Solaris 2.x supports filesystems upto 1TB, SunOS 4.x requires ODS 1.0 to support filesystems over 2GB. Solaris 2.6 and later support files > 2GB.
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 build 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.
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.