In a 36†wide by 27†deep by 40†tall grow room, is a 400 watt HPS bulb enough to flower 5 plants?
Thats less then 4' tall ... Sounds like your gonna have to LST the shit outa them... And as mentioned good exaust is key with a 400w in that space... good luck.
The bulb will be awesome in that space if you can air cool it and control your temps. I think you'll be pushing it with 5 though. I have a very similar size cab, and once the stretch starts, they either get tall, or wide. My 20x47 only holds 3 plants in 3gal smarties And a few 2L clones.