If it were me I'd do 3 nights at the Grand Californian. We've done 4 nights and we've done 3 at both resorts. I LOVED being at the Grand because it's SO amazingly close to everything & it really is beatiful, the rooms are peaceful & relaxing, and what other Disney resort in the WORLD are you able to exit your hotel INTO a park?!?! I absolutely LOVED that.
We'd go on Grizzly, get TOTALLY soaked, and run back into our room to change.
I've always thought 3 nights with a 4-day park hopper was a perfect amount of time. Believe it or not (and we go during busy summer time) by the end of day 3 we're usually really 'wrapped up' by then and trying to decide what to repeat again for the 4th day.
If you can get a flight that allows 4 full days (we arrived at 9am on Day 1, and left 8pm on Day 4) I think it's perfect. But if you get a better deal on 4-nights then go for it. Using points there is pretty 'expensive' so I always try to stick with the 3 nights.