Hi Michele!
My children, ages 5 and 10, have visited all of the parks since they were babies and they have enjoyed all of them (now and then). However, my 5 year old's favorite parks are the Magic Kingdom and Disney's Hollywood Studios. That's closely followed by Epcot, because he absolutely loves the Sea with Nemo and Friends Pavilion. His least favorite park is Disney's Animal Kingdom, though he does love several attractions there, especially Finding Nemo - The Musical and the Festival of the Lion King. With a 4 year old, I would concentrate on the attractions in Fantasyland in the Magic Kingdom and then move out from there; other favorites of my son are Buzz Lightyear's Space Ranger Spin, Pirates of the Caribbean, the Jungle Cruise, and Big Thunder Mountain Railroad (a tame roller coaster).
A "must do" for my family is Disney Character Dining. My children enjoy dining in Chef Mickey's in the Contemporary Resort the best, followed closely by the Crystal Palace in the Magic Kingdom. With a 4 year old girl, I suggest you consider dining at Cinderella's Royal Table in Cinderella Castle or the
Princess Storybook Dining at the Norway Pavilion in Epcot.
For your husband, the Walt Disney World Railroad circles the Magic Kingdom. It is about a 20 minute train ride on an antique steam-powered train if you don't get off at any of the stops. If your husband loves trains, then I highly recommend
The Magic Behind Our Steam Trains tour. It's a really great tour and takes place before the Magic Kingdom opens. Unfortunately, guests must be at least 10 years old, but perhaps you and your daughter could enjoy breakfast with some princesses while he goes backstage to see the roundhouse where the trains are stored and serviced.
Have a magical vacation!