Four days sounds pretty good to me.

: From my experience, three days for a first visit in the summer is a little too short.
BTW, you're WDW experience will be helpful for visiting DLR:
1) Go to the parks early (it's best to be at the gates at least 30 minutes early)
2) Take advantage of the Magic Mornings (whenever you get the privilege). MM lets you enter Fantasyland and Tomorrowland one hour before DL's regular opening time. MM days are Sat, Sun, Tues., and Thurs.
3) Use FastPass (at DLR you can also use the FP after the one-hour return time, until closing time)
Unlike WDW, park hopping is much easier at DLR. DL and DCA share the same entrance plaza (they call it the Esplanade), and you can walk from DL to DCA in just a minute or two. Instead of going to DL on one day, and going to DCA on another (like you probably would do if you were park hopping at WDW),
at DLR most people like to start every day early at DL and then hop to DCA later in the day. The general idea is if you start your day at DCA when it opens (10:00 a.m.), then you would miss the first 2 hours at DL. (FYI, the DCA gates usually open at 9:30 a.m., and they open Soarin' early.)
There's a lot of really good info in the "HydroGuy Tips Compilation" sticky at the top of this forum.
HTH