IIRC dedicated servers are only in Ranked, and you can't go into Ranked without all the DLC. The four map packs are the Horde Command Pack, RAAM's Shadow, Fenix Rising and Forces of Nature. Along with the free Versus Booster Pack, in total that gets you fifteen maps on top of the ones unlocked at launch. You also get 17 character skins, 14 weapon skin sets, new Horde fortifications, access to re-upping and Guardian, and 1000 extra Gamerscore.
Note, though, that if you're only planning on playing multiplayer, getting them separately would be a better option right now. Buying the Horde Command Pack, Fenix Rising and Forces of Nature while the HCP is on sale will cost 2000 MSP, as opposed to 2400 MSP for the season pass. If you don't care about RAAM's Shadow then it's the best option at the moment, but for an extra 400 points with the season pass you'd get access to a new campaign, the Chocolate and Liquid Metal weapon skins, and six character skins, as well as the achievements for it.