I think Las Vegas is all the same-old same-old you see on TV so you need only spend a few days there to get the full experience (because it'll be the same when you come back, just with different hotels). Eat, shop, gamble, and party. However, LA, Miami, and NY are great if you enjoy shopping (duh). For high end shopping, there's Maxfield in LA or The Webster in Miami, and just about everything in New York. Venice Beach is interesting if you're into the whole crazy people on the boardwalk-type scenery, but there's not much to do except walk down the boardwalk. Some stores on Rodeo Drive in Beverly Hills are interesting to look at, even if you don't plan on buying anything (like the Prada store). Do you plan on straying away from LA at all to Orange County or San Diego? There are a lot of sightseeing things in San Diego if that's your kind of thing. I've never actually done a lot of this stuff even though I live here but La Jolla, Del Mar, and Coronado are all very resorty if you ever want to just relax by the beach. They're also all very expensive places to stay, so maybe you're better off doing other stuff if you're going to 4 cities. I just couldn't not endorse San Diego here...
