I would say that four days at Disneyland would be a good amount of time to spend there. I can't help with accommodation, as we've not stayed at the Paradise Pier and we used our DVC points to stay in the Fairmont in San Francisco.
As for whether you rent a car or not, it really depends how happy you are driving. We did and went into LA on a Saturday and the traffic was absolutely fine - probably much better than a weekday. That's one tip I would pass on to you. I've heard the drive down from San Francisco to Los Angeles is a beautiful one, but we flew and got a really cheap deal on Southwest.
