The 4 inch fan/filter combo looks good, 170 cfm is about dead on for your cfm requirements for a 4x4x6 room...but you said you have the fan mounted in the top corner of your room blowing at your plants?You need to do the opposite. What you need to do is when you get that 4 inch exhaust fan is make a 4inch hole in the far corner of your ceiling, then hang your charcoal filter from the ceiling right at your 4 inch hole...buy some 4 inch ducting too, connect it to your now hanging charcoal filter then run it through the 4 inch hole you made and connect it to your exhaust fan , turn that bitch on 24 hours a day all day everyday sucking the air out of the room...try to completely exhaust the air outside, or in a attic or garage if you can, you DO NOT want to recirculate the same exhaust air over and over, fresh air supply directly from outside would be nice, but as long as your exhausting the air somewhere else youll be good. if your door has a good sized crack at the bottom that will probably be good enough for the intake too, when you install the fan you should be able to put your hand at the base of the door and feel suction.(not crazy vacum suction, just a nice gentle breeze.,...) if not you may have to make a tiny 2 inch intake(half the size of your exhaust).

peace