I'm guessing a 400W would be good for a 4X4 space. Years ago I had 3 400W in a smaller room 8X8 and I got over 2 lbs. Look for strains that are known to be easy to grow and have good yeilds. I ran into Sweet Tooth last Jan. and can't believe how it outgrows 4 other strains. So, your yield will be detirmined somewhat by how hearty and fruitful the strain is. I've had some strains that I'll never grow again due to their being kind of fragile and weak yield.
You should be able to squeeze in 4 plants if you don't plan on them getting real bushy.