Inferno 4 player = normal HP * 4.3. He normally has 6 or 7 mil.
As for how much DPS you need that mostly depends on class since the DPS number is just your base, which is then modified by your skills.
Barbs can get by with low DPS due to Wrath Quake or even just Wrath. Wizards kind of can due to Venom Hydra (which he didn't have). If you're spending half the fight dodging you need more DPS.
Overall melees need the least because they can just stand there and attack until he does his meteor swarm, then dodge, moving as little as possible (the more you move the more meteors he drops) then repeat.
I killed Belial with 11.1k dps. He didn't even hit enrage (although I suspect I could've tanked his enrage). I'm a barb, have 80k hp and 2.3k life per second, 9k armor and 550 all resists. Just used wotb (insanity rune), frenzy (sidearm) and hota (smash) for dmg. I'm basically able to tank everything but his explosions, although I can tank 2 or 3 of these without too much worry.