The Pats would save 6m+ on Parker, and 5.5m+ on Bourne.
If they sign Hopkins, I'd guess that once they get rolling through camp, barring injuries, might want to remove one of those guys, hopefully in a trade. Juju's going to get a lot of work, Gisecki's going to be on the field a lot, and they're not going to give up on Thornton after one season.
Or they could keep five - DHop/Parker/Bourne/Thornton/Rookie, but with Gisecki (who'll be in the slot a lot) four seems more likely. They only kept 2 TEs on the roster last year, but if htey're really featuring a 2-TE set, they'll probably need 3 this year.