Well they seem to have gone for "California King Bed," which is the albums only good ballad so it's a good choice. I would've went for "Cheers (Drink To That)" since after the 1st single but I think it is being geared up for a summer song release.
I agree with "Cheers" being the best single choice, but "California King Bed" is great. It's not necessarily a safe choice of single and I'm glad they went with it. "Cheers" should definitely end the era as the 5th single though.