As per the 4.1 maintenance version's changlog.txt a reboot is required for versions lower than 4.1.0.21098 that are being upgraded. If you are above that version, a reboot should not be required after you upgrade to a newer version.
After upgrading older versions of QAS a reboot is required or restarting all process with an open library to QAS.
QAS library in-use on Solaris:
N=/dev/null;for p in `ps -eo pid=''`;do pmap $p 2>$N|awk '{if(NR==1){l=$0};if(/quest\/lib/){print l;exit}}' ;done | sort -k2