monthly crontab entry

I would like to run a filesystem cleanup using crontab every 4th sunday of the month. I can see how I would set this up if I wanted to run it on the 1st of every month or every sunday but neither of these are exactly waht I need. Am I missing something ? Thanks
[282 byte] By [annib70] at [2007-11-25 23:34:44]
# 1
Make a script that that run every sunday, but add a check in it to see if it's the 4th sundaySomething likeif [ `expr \`date +%d\` \/ 7 \+ 1` -eq 4 ] ; then RUN_FILESYSTEM_CLEANUP; fi
y_p at 2007-7-5 18:19:26 > top of Java-index,General,Talk to the Sysop...