Hi Eric,
Thanks for asking your question here on the Disney Parks Moms Panel! Yes, with a single-day ticket to Disney's Animal Kingdom Theme Park, Magic Kingdom, Epcot or Disney's Hollywood Studios, you're welcome to leave and re-enter the same park as many times as you'd like on the same day. You don't need the Park Hopper Option to take advantage of this great benefit. My family actually does this quite a bit, since we like to hit the parks early and then head back to our hotel to rest and swim during the heat of the day before returning to that same park for more fun in the late afternoon or early evening.
If you're looking for more choices each day, you have a couple other options. You can add the Park Hopper Option to your tickets, which lets you visit as many of those 4 parks each day as you'd like. If the water parks and DisneyQuest Indoor Interactive Theme Park interest you, the Water Parks Fun & More Option might also be worth considering.
I hope that helps with your planning, Eric, and wishing you a magical visit! Please let us know if you have any other questions.
Beth