Kinda depends on what else you want to do.
If it's not a peak day, and you are organized, you can hit all the major rides in 2 days and have time for repeats...but it's hectic.
Three days, non peak, allows enough time to do *everything* at a good pace. Peak time? See 2 days above.
4 days at the resort would feel pretty laid back. But by this point, I would be done, I love the parks, DL is my favorite Disney Park, period. But the DLR is much, more compact than WDW and surrounded by one the the greatest, most varied, most diverse, most exciting parts of the entire continent. If I had to choose between DLR and the rest of the LA/OC area for my time...DLR would lose.
But, because the way the tickets are priced...5 day tickets allow you to mix and match with LA/OC. Get out and surf Newport Beach, hike the mountains above Malibu, get get a tour of LA, go to the observatory and the Autry in Griffoth Park, or LACMA, or tar pits, Disney concert hall, eat at Phillipes and the LA Farmers market, Long beach aquarium, depending on when in April see the Long Beach Grand Prix, bike from Venice to Playa Del Rey and back; etc. and then come back and have an evening in the parks. Other days, just enjoy the parks.