Hey there...
read my trip report... I just did USF / IOA, stayed at Hard Rock. I was ALONE with my 1 1/2 year old and my 4 year old. We had a GREAT TIME!
Some ideas for making it fun - I brought an envelope with quarters and pennies for the PENNY SMASH machines. There's a list of these somewhere on the net - can get it for you... This makes the shops fun without having to buy a big souvenir. Also, bring extra pennies to throw in the various fountains around ALL the parks (and Citywalk) (and even Disney). Both of these things made the walking around time fun for my guys. Even the 1 1/2 year old gets the throwing pennies in a fountain.
Anyway, here's a little more about the hard part of going with a 4 year old...
The first night we went on ET. Noah (4) freaked out in the line and screamed in the intro movie part. I know everyone thought I was a terrible Mom, but I made him go on even screaming. I knew if I didn't we would have nothing to do for the 3 1/2 days we were there!! I also KNOW MY SON and I know that he would be okay once the ride started.
You know your child better than anyone in the world, so you will have to gauge what rides to do.
FYI, Noah later (3rd day) freaked out in the WAITING AREA for BARNEY. I will be telling his girlfriends about this 15 years from now!!! Once we went in he was fine. Except he kept asking me if it was going to get dark, and I said "no" - couldn't imagine that it would! And then it got dark.
By the last night we went on the KONG ride and Noah LOVED it.
The Seuss area alone (In IOA) could keep a child busy for quite a while. Also, the play area in Jurassic Park. Strangely, my son thought that examining the dinosaur egg in the Jurassic Park Discovery area was the coolest thing. He also really liked watching the big boat take the final dive right across from there. Over at USF, the Curious George area is great (but very WET). The Barney show is right there, too.
Good luck if you decide to try it!
Please email me if you have more questions. I have done this and had a REALLY REALLY GREAT TIME!
Elaine