Dont listen to this guy, Look up 150 watt hps club on here. And you will see some very nice grows with 150's. 2 would be the best max amount of plants I would do. 150 watters can put out 16,000 lumes. More then enough for 2 plants. Look up Screen of green, LST, these are some ways to max the lumes the 150 watter will put out. If your looking to max your grow I would do one of those 2 styles. Your plants will also not stretch a ton if done right just like any other type of lighting. If this is your first time growing I would do a 150 watter rather then spend big bucks on the larger lighting. It's a great way to learn the plant and how to control the temps in your closet and such with out spending much. You can easily sell a used 150 watter for 50-60 bucks online if you want to up grade. But just my 2 cents.
The good thing about a 400w is, It wont get that hot in a room as long and you have some air flow. 400w is not overboard at all. Don't wast your time with a 150w You'll just end up wanting to upgrade anyway. With 2 plants under a 150w with some LST sure you might get some ok yields. 400w in at 4x4 closet is perfect just vent it have some airflow and your good to go. This is just my opinion.
To save your self some money buy a 400 watt HPS. You can run both M.H and HPS, you don't need to buy a special light to run both. It's a gimich to get people to spend more money, HPS ballast will run M.H.. A M.H ballast CAN NOT! run a HPS bulb. You'll kill 2 birds with one stone if you do it that way. I light my 2.5 car garage with a 400watt hps. These fuckers are bright! And they do push out some heat. If you could get a cooltube with a fan and pump out the heat that way.