New. Grow room help

I will be building a grow room in my cold room
4x6 I have a 400 w mh I can only vent a 2.5 inch pipe outside
will this be a problem? I am growing 4 plants. The fresh air pipe comes from the house in to the cold room. Will this set up work?

ur vent fan will have to run 24/7. bc if u have a a room thats 4x6 and if it has a 6ft thats 144cbft. and i have been told 3 is ideal for anything over 100cbft. so if u can try to get a 3inch hole if not run 2 2.5inch vent lines