Enjoy the off season while you can! We continued to travel in the off season until about 3rd grade. We would pick a time when school would already be out a day or two for teacher workdays -- and then keep kids out for the rest of that week. It's OK to miss a few days here and there while they're in kindergarten or first or second grade. BUT missing school days starts to become a problem when they're in third grade. (We did this with the blessing of a lot of teachers in our family :whoopie: )
Ocean Watch or any of the Hilton Head beaches would still be fantastic.
Great beach at Ocean Pointe and Oceana Palms (and there is a neat exotic animal rescue facility near there where you can see animals... a child would like that
http://www.mccarthyswildlifesanctuary.com/).
With a four year old-- Disney World is empty in September-- so you would not have to endure lines. Which would make it fun for you. I am a big fan of Legoland (Florida or California) for kids under 12 (actually, the younger the better). I would recommend Legoland for a day and one day of Disney World and then pool time at Lakeshore Reserve (remember you would also be able to use the pools at the JW Marriott).
Of course Newport Coast Villas would give you plenty of California beaches to choose from... and a day at Legoland. Also it would be within driving distance of San Diego Wild Animal Park.