This is a legitimate guide on how to see all four parks in one day. This is not a cheap ride one attraction in each park trick, or a no crowds needed way to take in a lot of rides. If you manage your schedule wisely and utilize extra magic hours you can take in a lot of great attractions in all four parks.
For starters, make sure you get a park hopper ticket. Seems like a simple thing to know, but make sure your ticket allows you to visit as many different parks as you like in one day. Also, it is key that you attempt this strategy on a day when the magic kingdom has late extra magic hours to maximize the fun potential. Having a car could be helpful but my wife and I managed this on bus and boat transportation.
Get a very good night's rest and start early from your Disney hotel and get to the gates of Hollywood studios at least 30 minutes before the park opening. 45 minutes to one hour ensures you are near the front of the line. Head directly to Toy Story Mania as soon as the gates open. Do not deviate from this strategy if you want to ride this ride. The line simply gets too long quickly. Skip the fast pass and go straight on standby and ride immediately. When you are finished head to the Tower of Terror and Rockin' Roller Coaster. Get a fast pass for the Tower then go ride Rockin' in the standby line. When finished hit Tower of Terror if your fast pass is ready, if not take in any of the other attractions you want to see, but get in the fast pass line as soon as your time is available. When finished with the Tower, head for the exit and hop a boat to EPCOT. The ride is about twenty minutes so you have time to catch your breath from the exciting morning.
Enter the International Gateway at EPCOT. Head straight over to Soarin and get a fast pass. Then take in an attraction or two with short lines. It should be close to 11:00am at this point and you can walk straight into World Showcase and be first in line at your favorite lunch place. After lunch your Soarin fast pass should be ready, ride it, and hit any other attractions you like, placing a limit of leaving the park via the International Gateway at 2:00pm. If a friendship boat is there, hop it; if not, walk. Either way head to the Boardwalk, walk through the hotel and out to the bus stop. Pick up a bus bound for Animal Kingdom.
Once in Animal Kingdom, go straight to the Kilimanjaro Safaris. Get a fast pass, then walk over to Expedition Everest and get in the single rider line if the standby entrance is too long. After Everest, take in any attractions you might like before your Safari fast pass. Once you have ridden the Safari take in any attractions you might have not seen yet or one more ride on Everest. When it gets close to 6:00pm head for the exit. The best strategy at this point is to take a bus to the Animal Kingdom Lodge Jambo House. Exit, stay at the bus stop, and catch a bus to the Magic Kingdom. An alternative that is a bit more adventurous and moderately longer; catch a bus to Boardwalk, walk or boat to EPCOT, walk out the front of the park to the monorail and take it to the Ticket and Transportation center. From there, board the Magic Kingdom monorail or ferry boat. I would only recommend the latter strategy to someone who considers a monorail ride part of the experience, or needs to catch a later fast pass time for Soarin or another EPCOT attraction they missed in the early afternoon. It is also a great way to grab dinner if you have a reservation at a restaurant in World Showcase.
If you made your adventure on a day with late extra magic hours at the Magic Kingdom, you should be showing up at the gates between 7:00pm and 8:00pm with four to seven hours left to ride EVERYTHING in the Magic Kingdom. I have seen summer weekends with EMH until 2:00am! A couple suggestions on getting around the evening crowd in the Magic Kingdom. Work clockwise around the park. For some reason, the crowds seem to go that way in the morning, and by bringing up the rear you are not facing into the rush. DO NOT get stuck on Main Street after the Electrical Parade or Fireworks. Watch those from a different location so you do not get caught up in the mass exodus. If you rested up the night before and make it until 2:00am, you will have time to do everything.
Looking back, you should be able to say that not only did you go to all four parks in one day, but you managed to see the major attractions at all the parks, and have a great time doing it.
Some other last minute tips:
Hydrate with plenty of water. Save the sugary stuff for late in the evening for your last rush.
Do not panic early in the day, you have plenty of time. I like to think of my four in one day schedule to be like this:
(8)9:00am - 10:30am Hollywood Studios.
11:00am - 2:00pm EPCOT
3:00pm - 6:00pm Animal Kingdom
7:00pm - close Magic Kingdom
I like the Magic Kingdom the best so I spend most my time there. If you like one of the other parks, extend your time there and get to MK later.
With good strategy one can make a legitimate visit to all four parks in one day.
GOOD LUCK!