My mother and I just returned from her very first trip to the World. We only had 3 days, but she wanted to see everything....and we did! Using early entry, we did this:
Day 1: EE for MK (we did Tomorrowland and Fantasyland), left around 11:30 for Epcot (we did all of Epcot except TT and MS, and stayed for Illuminations that night....we even had two PSs: one for Garden Grill for lunch and another for later dinner at Le Cellier in France, so we started the World Showcase in Mexico)
Day 2: EE for AK (we did EVERYTHING here except Asia by 12:00), left for Disney-MGM (had a PS at Prime Time for late lunch), did most of the park that afternoon, stayed for Fantasmic, and then went to Boma for a late dinner.
Day 3: Spent all day at MK. Entered the park early for an early PS at Crystal Palace. We also had an early dinner at Liberty Tree Tavern. Watched Spectro and Wishes.
So you can do all four parks in 3 days, if you are using Park Hopper tickets. Course, I've been several times and had the good stuff picked out! We only did Character autographs at our 3 character meals, and that saved time in the parks. We were going at a pretty good clip, but still fit in a lot of fun and plenty of "retail therapy" (aka: shopping!). We also used Fast Passes wisely, which also can cut down on wait time, and rode those rides which build up a wait time quickly earlier in the day, if we could.
Have a great time, whatever you do!