Venting My Lights, Help please.

I need some ideas on how to vent my lights from the basement. It seems like all the vents are standard 4" and i'm trying to vent out through 8" ducting. I know about reducers but I just feel like it wouldn't do a good enough job cooling my lights if I vented the intake and exhaust through a reducer since the whole gets so much smaller. Any Ideas? "by the way im trying to vent through the standard block windows" . . . Thanks.

If it is in your basement, I would drill a 6" hole just above the foundation wall (or out the window, just use plywood to fill the frame, and drill hole). They make outside dampers for kitchen range hoods in that size, and are easy to find. That way it would look normal from the outside. I find that going from 8" to 6" is not a problem for overall airflow, but does increase air speed.