You can grow 4 plants with a 400 if they're not huge plants. If you scrog you can have more than 4 even. I've seen as many as 10 in a scrog with a 400.
Personally I run a 400 with a Supernova hood in a 2'X4' room and have 2-4 plants at any given time it seems. There were 2 small ones when I got the light and I put 2 more in there that are pretty big but then harvested the original 2 a couple weeks later so there's only the 2 big ones now.
Just noticed you said MH. So if you plan on only using it for veg, you could do a lot of plants with a 400 as they will not be very big until flowering.