We did it in October in 6-7 hrs & did the tour that included the Astronaut Hall of Fame. We had 2 kids (18 & almost 13). They liked it, but were a little bored. However, afterwards I would see them saying "we were there" when they would see an advertisement on tv. Also, they now know things about the space program that they had no clue about before. They really enjoyed the movie & the AHof Fame. We could've spent a lot more time at the AHF than at the Space Center. My husb. & I would love to go back & spend more time, but for the kids it was more than sufficient.
The tour that we took is mostly indoors & air conditioned, although you travel on a bus to each place. Basically, you can be there as long as you want to. Four hours could work if you want it to, but as was said, it is fairly expensive, so you might opt to stay longer just to get the 'most of your money'.
My husb. & I really enjoyed KSC & I would recommend going there as opposed to a park before the cruise (since you'll be in one afterwards anyway) because it will be much more relaxed & not as hectic as pace.
Universal is nice, but a lot of walking, rushing & also an expensive day. Personal preference, I guess.
I like the idea of 'hanging by the pool'. You could go to Downtown Disney & sight see, also if you wanted to venture to Orlando. Also, there's Sea World, Busch Gdns, etc.
Have a great time.