Ooh, that's tough. I'm going with the four day base because you've never been to either HS or AK and I think both parks are worthy of a full day to explore for a first timer. in HS's case, it's really tough, there are a lot of big rides with long lines that can eat up your day. So here's my idea:
Wednesday: AK day. You'll need to get to the expo at some point, probably the first thing you do? So knock that out first and then spend the rest of the day at AK.
Thursday: HS. The 5k will allow you to get back to your room, shower and change, and get to HS before early opening if you want (and I recommend it!). I did this this year and was able to knock out quite a few rides (starting with Slinky Dog right at opening and including a Rise LL) before lunch.
Friday: Make this your MK day
Saturday: DS and pool, as you mentioned
Sunday: Epcot