You will barely touch the surface of WDW with four days! You definately will wish you were there longer!
Your kids will like Epcot. There is plenty to do in Furtureworld and then they can get their passports stamped and signed at every country. We find that we really need at least two days to do all of Epcot. Make sure you include Illuminations at the end of your day at Epcot; the fireworks are spectacular!
There are two other parks you did not mention over and above MK and Epcot and those are the Disney Studios and Animal Kingdom. There are plenty of things to do at each park. Your kids will like those, too. Don't forget Fantasmic at the Studios; it is a wonderful show with fireworks, fire, animation on water sprays, etc. It is wonderful!
You should also plan on a few character meals for your family. The characters will come to your table to sign autographs and take pictures with your children. It sure beats standing in long lines to see the characters within the parks. Some of our favorites are Crystal Palace with the Winnie the Pooh gang (Pooh, Piglet, Tigger, and Eeyore), Cinderella's Castle breakfast with the princesses (but you have to make reservations exactly 60 days prior within the first three minutes of opening of WDW Dining; you can call 48 hours before and check for cancellations), Donald's Breakfastasaurus (Mickey, Pluto, Donald, and Goofy), Hollywood and Vine (Goofy, Chip and Dale, Pluto, and Minnie), and Chef Mickey's (Mickey, Minnie, Goofy, Chip and Dale).
I do believe you save money with the hoppers vs. paying each day, plus you save time in not standing in lines for tickets each day.
You will have a magical trip. Have you gotten any of the guides you can purchase at book stores? You might want to get one so you can read up on all the rides at each park and get an idea of what each family member has an interest in so you are not spinning your wheels in the beginning of the day trying to decide where to go first. You can also find the guides at your
Disney Store. The books are very helpful for the first time visitor. I went the first time pretty niave to what all WDW offered and my husband and I felt like we spent a lot of time figuring out where to go and where to eat and feeling kind of frustrated when were were doing a lot of walking and not a lot of riding.
Okay, I am long winded....PM me if you have any more questions. I have not stayed at PO myself, but have heard good things about it. Have fun planning!