It would probably be a good idea to turn on your lights for several hours and check the temps in your grow room before you need it. I just got a 4" inline fan just like yours and I'm real disappointed at 40cfm amounts to. It's ok for an active intake in a small area, but it sucks for exhaust. I'm running about 410 (true) watts of cfls in a 16 cu.ft. box and the 40cfm didn't do squat as an exhaust fan. You'll have more heat and a bigger area to vent, once you fire up the HPS, so you'll probably find that you need a better exhaust fan.

I'm on my first grow too, a few steps ahead of you, and the first lesson I've learned the hard way so far is that these people are serious when they talk about the importance of good ventilation. I hope I'm wrong and that the fans you have work, but it's best to test them now while there are no plants in the grow area and you have time to take remedial measures.