@ryman471 I think before you decide how many days to go, you need to decide if/when you'll be back. If this is going to be a once every 5 to 10 year trip or one and done, do more days at the parks. If you think you can swing every 2-3 years, you might be able to make 4 days work for a first visit. My first visit we had 2.5 park days. Spent 1.5 at MK and a full day at what was then MGM studios. Were those 2.5 days "enough" for us? I'm not sure and I didn't know at that time when I would be back.
I do agree with
@Mome Rath about the kiddie rides being the most important parts of Magic Kingdom. My trips are not complete unless I do the "classic" rides. In fact I'm still kicking myself that we couldn't make Pirates of the Caribbean fit in the schedule on our last visit. I learned my lesson, PIRATES IS MANDATORY or it's not a real trip for me.