First--yes, you have enough light. You've probably got around 21000-22000 lumens for a 4 sq. ft. area. Plenty. A 400w HPS would be overkill; a 250w would be more than enough in that area, if you chose to go with only a single light. No matter what, paint your walls flat white or use mylar on them. Get everything you can out of the light you've got.

stinkyattic is very much right, IMO. Ventilation is almost always the biggest issue in smaller grow areas. It's why I personally advise people, especially first time growers, not to get more lights than they need until they know they can keep a room at the right temp. You'll definitely need some sort of fan and/or ventilation system in a room like that. Plan for that before you try anything.

3 to 4 plants? Your area may be a little small if you go for 4 plants. Maybe others have different grow methods, but I always figure on 1.25 sq. ft. per plant. Even with that, they'll grow into each other. One sq. ft. for a plant is cutting it close, IMO. On the other hand, if you're in a 4' tall closet, that may be okay...just don't let your plant get too tall before switching to 12/12.