If there's larger explorable areas with more interesting content then four planets should be fine. Saying it just has four planets when you don't yet know how much there is to do on those planets is a meaningless statement. There's also strikes, which the one they've shown off (Inverted Spire) looks far more fun than the strikes in D1.
And before I get labeled as a fanboy, I'm not defending D2, it could very well be that Bungie has learned nothing from D1. Just pointing out that already claiming there's going to be a lack of content is a big ignorant. Hell right now the total size of D1 with all of its expansions is 60gig, and the base game for D2 is 68 gigs. Sure some of that can be attributed to higher res textures but Bungie has already stated that one of the patrol locations is easily twice the size of the largest patrol location in D1. And they've already mentioned there will NPC's in the patrol areas that give side quests (with side stories) as well as random Dungeons you can stumble upon. And they recently announced something called "Flashpoints".