This will give you a list of users and gid starting with 4 - modify it for your needs ( sorry it's what I use to write scripts in).

#!/bin/csh
set allusers="`who -q`"
foreach xx ($allusers)
        if ("$xx" == "#") then
                 exit
        else
                # find info on user
                set mygid=`grep $xx /etc/passwd|awk -F: '{print $4}'`
                if ("$mygid" != "") then
                set first=`echo $mygid|awk '{print substr($0,0,1)}'`
                        if ("$first" == "4") then
                                # let me know who
                                echo $xx"  "$mygid
                        endif
                endif
        endif
end