Using hps it’s recommended that you need 40 to 50watts per sq ft, a 4x4 has 16sqft. To adequately light that with a hps you will need 640watts as a minimum. It would be a bitch to vent (2) hps lamps in a 4x4. Have you considered quantum boards? - - - - - - - - - - - - Come & check out: https://forum.grasscity.com/index.php?threads/GA’s-grow-&-chat!.1562064/
I wouldnt put more than 1 plant under each light although that may be pushing it come flower time and yield may suffer a little! If you can maintain temp in the tent I dont see any other issue with that. Plenty other people will argue and tell you you should ditch them and go to quantum board led lights. But if that's what you're using then grow under what you choose! I'm running state of the art (well they were 5+ years back) Mars II 400's.... If you look about on ebay there are a few sellers that have 2for1 deals on them clearing old stock! Not cutting edge but would be a a very cost effective upgrade! Snoochie Boochies!