Hi Joni!
Oh, you are going to have so much fun! 4 and 2 are perfect ages for Disney World. But I know it can be overwhelming to have them out of their comfort zone for so long!
The first thing you need to do is realize you are NOT going to see everything while you're there. Disney is just too big and too cool to see in one week---or even two! So, don't worry about trying to push push push and make sure you see everything or that the kids experience everything.
Take time to go back to the hotel for naps. Explore your hotel's recreation offerings. Do the playgrounds at the parks. Stop by the
Baby Care Centers and relax for a minute. Sit on the side of the road and eat Mickey Bars without even looking at your map. The real fun at Disney comes when you just EXPERIENCE it. Like you said, don't overwhelm yourself---just have fun!
I do think 2 parks is a good idea. Especially if you're just staying for a couple days. But if you are staying several days don't be afraid to just go to all the parks--one park per day and just hit the highlights. One of my very favorite things Disney has to offer is the
Magical Beginnings map. You can download it and see all the best attractions for little guys your age.Then you can set a loose schedule and not overwhelm yourself when you walk into the magical-ness that is Disney World!
If you have more specific questions about your kiddos or vacation (resort? length of stay? park choice?), please ask me again. I'd love to help!