If your intake is passive you are going to have to go bigger. Why 4", I have not seen to many 4" reflectors these days. If your fan is kicking you may need a "router speed control" to slow it down a bit. There is a point where you begin to pull to much air through the carbon filter and it becomes ineffective. You will also need a fan inside the room moving air, a small fan blowing across the light really lets you bring it in close. Your setup is similar to mine, check out my sig. I have a 2nd squirrel fan modified that has really helped. GOOD LUCK..
I would try to create a vacuum if your operation will be sealed. Pull more air out then intake, this will create the sucking effect and if you scrub your air, by the time it exits, it should be all clean. Not everyone has a lot of space, but I would exist the air to another room, scrub it there again, and then exit out the house, incase you come home one day and it fails, the backup will scrub it. A little dash of ozone will do the final scrub, put ozone on a timer.
My bad. I forgot to mention that the cool tube is a 6'' with 6 to 4 inch reducers on it on each side for the tubing. I also have a small fan by the intake to move the fresh air that I forgot to put in the sketch. Thanks for the replys. Much appreciated. Will Have some pics up very soon!
Try to keep that 6" all the way. If you reduce it from 6" to 4" then it's the same as if the whole thing is 4".
Personally I would separate the light venting from the room like I have it in the photo below. You also want to push the air through the light tube as this will create positive pressure inside the tube/ducting and will force fresh air into the room is there are any small holes in the tube/duct system. (I don't have that set up ATM but will once I get a more power full fan If you suck air out (easier on the fan) you could potentially suck unfiltered air from the room into the tube and then out if there are any holes in the ducting. http://www.cannaculture.com/photos/p2030152.jpg
I agree with toastybiz. If you can keep everything 6", your fan will pull much more air. I'm not sure about the Vortex brand inline fans, but for EcoPlus, the 4" costs $135 and moves 170 CFM. The 6" costs $175 and moves 440 CFM. I think it'd be better to wait the week or two if you're on a budget, and go with the 6" fan. Since you're going to be pulling air through your carbon scrubber, then over your lamp, you will want to have as much juice available as possible. Fan upgrades aside, I really like your cab setup! It's pretty similar to mine. Post some pics for us to see