Tickets are not specific to a park -- but you must have a valid ticket to make a theme park reservation.
I think what you are asking is can you "hop" after 2pm without going to a different park first. The answer is no. You must have a park reservation at 1 of the 4 theme parks, and tap in at that park, before you may hop and enter another park (without a reservation). If your "after-2pm" park does not have availability for a reservation, you should make a reservation at a different park and tap in there first.
But keep checking the theme park reservation availability -- and actually try to make the reservation -- because people change plans all the time. You may get lucky and be able to grab the park reservation you want.