I fish a 6 due to how and what I fish. Heavy shooting head lines and clunky flies (think clousers and cone heads) a good chunk of the time. I actually fish a 7 for coho a good amount for the same reason (bigger clunky flies) plus heavy sink tips on some of the deeper, more exposed (wind) beaches in MA 9 with strong current.
A 4 or the 5 can efficiently handle the fish we catch in PS with a few exceptions. Some beaches, a 2 or a 3 would be more appropriate considering some the SRC you will be catching. Not the only thing to consider though. Personally, I also don’t care about getting the most out of a fight with SRC or coho for different reasons. SRC I want landed and released asap. Coho I want landed asap and cleared of other anglers, especially on crowded beaches. Releasing wild coho fast is important as well.
If the plan is to fish a floater or intermediate 95% of the time, the 5 will be fine. If not, consider heavier. It does not need to be expensive. Lots of options out there from Echo, TFO, etc. will work very well. If you want to bump to mid range, GLoomis Pro or even the Sage Maverick are solid rods for the sound. If you are poor about gear maintenance like me, consider something with saltwater components.
To me lines are as, or more important to your setup than the rod. See the beach lines thread for lots of good info there. Most recos would apply to fishing from a boat as well.