For us, we'd probably do the 4 night on the Dream if we were combining it with something else, which we probably will never do again. Some of us, not me, but some in my traveling party that may be related to me by marriage, get restless if the vacation is too long. I don't get it, but they do.
We've done a 6 night WDW vacation combined with a 3 night Dream cruise. It was apparently "too long" and the cruise part was too short. Barely enjoyed it. I think I took a bunch of naps on the cruise to recover from the WDW portion and didn't enjoy the ship enough.
We've done a 4 night Universal trip combined with a 4 night Dream cruise. The cruise was the best part of the trip and we enjoyed it, but we'd never combine it with Universal again. Guess we're just Disney people. We didn't even go to Universal the last day because we felt we'd seen it all. Plus, this combo was by far the most expensive trip we've taken because we stayed at the Hard Rock and didn't use DVC points.
We've done a 7 night Fantasy cruise and loved it. We've done a 5 night Wonder cruise and loved it. I liked the 7 night cruise the best out of all of this, but I think my wife liked the 5 night better. Both of us enjoyed the stand alone cruises more than combining them with something else.
So, it all depends on how long you're wanting to go on vacation and what type of vacation you want. A WDW trip is awesome. A
DCL cruise is awesome. But they are different. Combining them changes your mentality considerably in the middle of your trip. Even our "no kids" WDW trips, which are far more relaxing with mid-day breaks we usually don't take with the kids, are very different than a cruise. We are very goal oriented WDW vacationers. We understand we aren't going to do everything there, but we go into it saying we want to make sure we do XYZ. Cruises are way more mellow for us.
Even with all of that, I'd take the 7 night cruise and my wife would take the 4 night, so we're no help to you at all!