I know I'm in a major minority here, but with that short of a trip & being the first trip, I would not start with MK. MK is "the" park of WDW and all the other parks might be a let down for your 7 & 6 yo if you start with MK.
If you are going to do all the parks, I (personally) would start with Epcot, go on to AK, the MGM and end with MK. Epcot has the least going on as far as rides. It can be a big let down/bore to do in a short trip if you do it later. AK (my kids favorite park, btw) is a nice switch, a few rides, but more shows & exhibits. MGM will start to introduce them into more "going-ons" and MK is the crowning glory.
I wouldn't attempt all 4 parks in only 4 days. I'd probably skip Epcot in a short trip (least for kids & my kids least favorite park). I'd do AK first. If you have park hoppers, I'd then finish with MGM if you have time (you can do each of those parks in a half day if you don't savor what is going on). Day #2 I'd do MGM. If you hit MGM on the first day also, do a half day and then move on to MK. Day #3 would be MK and day #4 would be going back to whatever park your kids want to experience again. Probably MK, as that has the most & it really takes 2 days to do it well.