Well you need to talk more about your set up. With 400w hps in a small tent or closet you will need a cooltube and a extra fan just for the lamp.
Or the plants will fry.
Happened to me like 2 years ago.
The cooltube makes it ok, a bit on the high temperature but ok for the plants.
Also for the ppfd you should read your meter on the direct sun light, for me is around 600ppfd and try to match it inside.
A led lamp also takes care of it, but they are pricey but I tell you that they worth every cent.