Hi there
We're going to be there the same time as you! We're staying at Coronado Springs, but POFQ was also at the top of our list.
I'm sure the drive will be fine for you. Especially on the way down if you're driving overnight. We drive to Hilton Head, SC every March - its about an 18 hour drive for us. We've found that taking a break, even a quick one, every 2 hours is the key. Our girls are older than yours so we also rely on movies and electronic games (DS and Leapster). Your plan to have a bag full of surprise gifts is also a good one
Of our 5 sit down meals, 4 of them are ones you are doing too! None on the same days though. If you know your girls (especially DD4) are going to be good with meeting the characters I personally think character meals are a great time saver. You have to stop and eat anyways so you might as well meet some characters at the same time and then you don't have to wait in line to meet them in the parks.
Your daughter will love BBB if she already likes to dress up. Our girls did this on our 2009 trip and we all had the best time. One of my favourite pictures from that trip is of my DH watching them be transformed - he has the best expression on his face being excited for them

We also ate at CRT the same day which was like icing on the cake for the girls, there is something magical about eating in the castle!
As for your concerns about DD4 going on some of the more thrill type rides, I think you'll quickly get a sense of what she can handle. On our last trip our younger DD, who was 4 at the time, went on more thrill rides than her older sister. Those included TofT and Splash Moutain. Both my girls loved Soarin' and Test Track. My girls definitely found it helpful to watch ride videos on
youtube ahead of time
Regarding specific plans, we will be at Epcot the same evening as you and we will also be at
MVMCP on the Wednesday...maybe we'll run into each other!