Hi Kimberly! I am so excited you are planning a Walt Disney World vacation with your special four year old. I know you will want to make it beyond memorable and I am sure it will be.
My favorite suggestion for the most magical Mickey Mouse meet and greet is seeing him at the
Magic Kingdom Town Square. Here you will have the opportunity for a personal one on one meeting of Mickey behind the scenes in his dressing room. The best part is that Mickey will actually talk to your child and it makes it far more personal then other meet and greets. He spends plenty of time with each guest and there is a PhotoPass photographer on hand to capture each moment. This is a FastPass option and I highly recommend you use one for this experience.
There will be many character meet and greets throughout the parks and I can guarantee that each one will be extra special for special needs children. Whether it is a princess or a Pluto they will take extra time to make the children get the experience that they have dreamed about.
Also consider
Character Dining while you are there. Reservations are a must and do this as soon as possible up to 180 days in advance. During these meals the characters will all come to you at your table and stop for a photo and an autograph. They must keep moving but they spend as much time as possible to make the event exciting for the children (and adults!).
Although there are no special needs specific character meet and greets you will find many super opportunities to make this vacation extra magical.