Can we make it work and if I buy a 3 day Park Hopper and end up going Wed can I add a 4th day while ther for Sat?
You can upgrade your tickets (add a day) as long as you do it before the day ends on the day you use the last entitlement. So if you go in on Wed, Thur, and Fri, you will need to upgrade the ticket on Friday before the parks close. Plan on paying the diference between what you paid for the ticket and the gate price. You may get the ticket price bridged, but it's not guaranteed, so it's best not to expect it.
Is Thurs. Fri. and Sat enough time?
It honestly depends on what you mean by "enough time" and how much you want to get done? If you want to do everything (as in all of the rides and see all of the shows), then no, I don't think that's enough time. I think you can get a lot done, but you will probably have to forgo some things.
It does also depend a lot on when you are going. In the summer, in the fall? When specifically are you planning on going. In the "off-season" the parks run with reduced hours and entertainment. In the "peak-season" the hours are longer and there is more entertainment, but the crowds are also higher.
I don't want to do the best stuff first so I was thinking starting low key with the kids by doing the boardwalk at DCA.
Do you mean on Wednesday, when you arrive? You can certainly do whatever you want first. The Boardwalk area at DCA has some rides, including TSMM, which can have a very long line. There are some other cute rides in the area.
Otherwise, I would make plans to hit some of the big rides (especially in DCA) at rope drop (assuming your not going to EMH/MM). Things like TSMM and RSR (or RSR FP) have the shortest lines right at rope drop. One common strategy is to ride TSMM right at rope drop (again assuming no EMH) and then get a RSR FP. Otherwise, you're looking at a 45 minute wait for TSMM.
I would not hold on riding "the best stuff" because if you wait and the ride is down when you go to ride it, you might miss out. At least if you start out early in the trip trying to ride the big rides, if they are down, you have time to go back later. Rides like Indy seem to be down frequently, so if you leave it until the last day, there is a good chance you'll miss out on it.