I always enjoy visiting both. They are different and my family enjoys having options. We do stay on site at Universal for a few nights as well, but, it certainly is not necessary. You can cab over to Universal (the easiest,maybe not the cheapest) or maybe you will have a car. This board offers many ideas regarding methods of transportation if you decide to spend time there.
The four year old will love the Dr. Seuss area, and Islands of Adventure in general is a teens' paradise and an adults' playground. Yes, you will get wet!