a quick question

how big of an HPS/MH light will i need for a 4 ft x 4ft grow room?

or how many plants can i put under

1x 400w
or
1x 600w?

mainly im looking for the size of hps light ill need for 1/msq

4 foot by 4 foot I would go with 600 watt, or 1000 watt with really good ventilation.

a 400 watt can do anywhere from 1 to 36 depending on how long you veg/ the strain/ size of pots
a 600 watt can do anywhere from 1 to 64 depending on how long you veg/ the strain/ size of pots

If you're using HPS I assume it's for flowering, therefore you want more light. For 1 meter squared I would go with a 400 watt, or 600 watt with very good ventilation.

600 at least