Although I think it's a bad idea and it would be better to run it every 14 days, this should do what you want. (To run at 4:00 at night)
0 4 1-7 * * test $(date +%u) -eq 7 && ./yourscript.sh
0 4 15-21 * * test $(date +%u) -eq 7 && ./yourscript.sh
0 4 22-31 * * test $(date +%u) -eq 7 && ./yourscript.sh
**Edit:** I forgot to add minutes -> fixed