Hi Britta!
Congratulations on your first family vacation to Walt Disney World! I am wishing you all a very happy time together in this special place.
I would recommend that you go to the Magic Kingdom first. In fact, you can't go wrong if you chose to spend all four days there, especially with a 4 year old! And, to top it off, they just opened the amazing New Fantasyland area, which is full of amazing things to see, do, and explore.
If you do decide to visit another Park, I would recommend Disney's Hollywood Studios second. There are some wonderful attractions there for the young ones, such as Toy Story Mania!, Disney Junior-Live on Stage!, Muppet Vision 3-D, and the Voyage of the Little Mermaid.
I suggest you plan your days so that you have a break time in the early afternoon, which is usually the most crowded and hottest part of the day, not to mention the time of day when my 4 year old tends to need a nap! If you are staying in a Disney Resort, head back for naps or a dip in the pool before heading back to a Park or dinner.
As for a
Disney Dining Plan suggestion, I would suggest that you read over the different plans on the link I just posted and choose one according to your family's style of eating. Do you prefer Table Service meals every day? Once a day? Twice a day? Or, is Quick Service enough to satisfy? Our family usually chooses the basic Dining Plan, which allows for one Quick Service, one Table Service, and one Snack to be used at our discretion throughout our stay. If you wish to attend a Character Dining experience, make sure to make your Advanced Dining Reservation up to 180 days in advance (407) WDW-DINE.
Have a wonderful time!