Hola, Sylvonne! Let's get on that planning horse and help get you on your way!
You'll find that there are many attractions that your 4 year old son and 11 year old daughter can enjoy together. And, if your daughter is into girly stuff, your 4 year old likely has not likely developed an aversion to princesses and pink just yet.
One of my favorite family activities when planning an upcoming trip is to gather 'round a (Mac) computer and look at attractions. Grab your son and daughter and
visit the maps section of the Disney World website. Through this page, you can create your own customized maps. Your son and daughter can get a good look at all the attractions, find that they have many similar interests in attractions, and then plop those attractions on a map. You can order keepsake-quality maps and have them delivered to your house for free.
In addition, your 4 year old is likely 40" or taller and will be able to take part in most of the thrill rides with you and your daughter.
The last piece of the puzzle in terms of attractions is how you go about enjoying the attractions. Take a good look at the attractions you selected on the map and then devise a good touring plan to take it all in. It's especially important to be methodical in your approach to visiting attractions because you'll be in town during peak crowd levels at Easter time. The best approach is to arrive early to a park, hit the most popular attractions (in a logical order) first. The website TouringPlans.com can help you get specific.
Giddy up!