Why are you set on doing all parks? Will this be your only trip to Orlando ever?
If you figure ~$80pp/day times four parks = $320pp for tickets
A 4 days Disney ticket is $247. 5 days is $252. 6 days is $258.
My advice? stick to Disney for this trip. Buy the 6 day tickets - on the "relaxation days" go to a park for 1/2 the day or just after dark when a park is open until 10 or 11pm.
I think you mentioned the beach. Remember that the beach is an hour from Orlando. That's a lot of time and gas. Your kids would probably have more fun riding Splash Mountain or Kali River Rapids multiple times.
If you are dead set on trying to do MK, Universal, and SW, please let us know cause you're going to get a lot more posters trying to convince you otherwise.
My guess is that the OP is trying to do Disney AND Universal to appease her older teens that are 17 and 19 years old. Most teens that age want to do the thrill rides at Universal. If this is their only family trip to Orlando for awhile, I can see why the OP is trying to do both.