I think 4 days is more than enough. Honestly, I think 2 days is enough if you have other things to do in California. You can do California Adventure in one morning and then DL in 1 1/2 days. Of course this depends on crowds.
We went for 4 days and had to do things 2 or 3 times just to keep busy. It was very different than WDW where you have endless places to go and things to see and have little opportunity to get bored.
I would suggest seeing many parts of California and explore Reagan's great state.
We traveled from Sacramento to LA in 7 days ending at DL.
If you need any advice on the places we saw let me know. Here's our trip report.
http://www.mousebuzz.com/forum/trip-...adventure.html