Most frequently, they use Ch. 52 Chester/Fairfield 21392. However, Troop 4 dispatchers regularly use patches on the SCHP dispatch talkgroups when a single dispacthers has to cover multiple channels simultaneously. If you don't have a scanner that properly handles patches, you may miss radio traffic depending on how the patch is set up. I would recommend programming all three Troop 4 dispatch talkgroups into your scan list/group and scan all three until you confirm which are most commonly used.
The other two Troop 4 talkgroups are York 46128 and Lancaster/Chesterfield 21104.
21520 is now a Troop 1 talkgroup, though it used to be used in Troop 4 several years ago.