You should use the 4" vent fan to ventilate the room heat and air out. You should have the 6" vent fan hooked up to air cool the 600w light. Get a small booster fan or squirrel cage blower for an intake fan, you don't need too much. If your reflector is not air coolable, you will need one, there's no way around it with heat problems. Your plants will be happy, your temps will make you sane and your bulb will last longer too.
Like dadio161 said, no curves. One 90 degree curve will cut air flow by 50%. If you have to put a curve in there, but a duct booster fan in, that is what the are made for, increasing air flow in corners.
Mount a small 4" filter on the 4" exhaust and hook up the 6" either with a fresh air intake from outside and a hot air exhaust for no smell. If a fresh air intake is out of the question you will need a second carbon filter on the open end of the AC reflector to combat odor from there. Buy a real filter, dont make it. Manufactured filters are compacted so much tighter than a home-aid filter, they last longer and are more effective. If you want to save money on a filter, buy filter made by a distributor, not a Can Filter. Eco Plus from Sunlight Supply or Active Air from Hydrofarm work and are literally half the price of a Can Filter.
If you don't have vent fans, you can save money buy going with a distributors brand again, like Active Air, Eco Plus or Sunleaves. You can again save half off a can fan and get a great quality fan. If your really looking for the best fan, don't get a Can Fan, get a Vortex!