3 is sufficient but 4 is better for someone who has no plans to come back. In fact for a fanatic, 5 might be worth the added cost depending on how much it is to add a day (sometimes it is really cheap). I wouldn’t recommend 5 whole days and certainly no more than 5.
I always recommend hoppers but especially for 3+ days. It’s a flat price to add hoppers regardless of number of days so it’s especially worth it for longer stays. The parks are so close, even more so than USF / IoA. Hopping time is 11am and after that you don’t have to tap into your APR park so it really makes your plan much more flexible.
If you can swing Universal toward the beginning (or end I suppose, but that would be a downer IMO), I would stay somewhere closer to USH. I don’t know much about the hotels in that area. Do any have Express Pass privileges? If so, definitely do that! Also, be there for early entry for MarioKart. Make sure to join the Virtual Queue to get into Nintendo World if they’re using it when you enter the front gates.
This is really a personal preference thing — there are beaches (Huntington is probably the easiest to visit), other studio tours, celebrity tours, Hollywood Blvd / Walk of the Stars / Graumann’s Chinese, Griffith Observatory, Rodeo Drive, La Brea Tar Pits, Forest Lawn Cemetery, and many many museums and other sites that you may enjoy seeing depending on your interests.
Oh I recommend eating at Duke’s in either Huntington or Malibu and get the Hula Pie for dessert. 
Peruse this thread for more info comparing WDW to DLR:
Feel free to ask more questions if you have them!