I have a very similar setup to you. I have a 4x4x6 foot Secret Jardin w/ a 600 watt digilux bulb putting out 95,000 lumens and a lot of heat.
-To combat the heat I have a 400 cfm exhaust fan (duct fan). This is hooked up to my light, pulls air out of the tent and through the carbon filter. I use a controller to run the fan at about roughly half speed.
- I have one passive duct that sucks air in through negative air pressure. This is near my window air conditioner where the cold air is.
-On nice fall days when the temps are in the 70's, I don't use my AC, just a window fan to bring in the air (cheaper than a 500 watt ac unit).
-My temperatures in the tent remain very stable, usually 2-5 degrees above room temperature.
Make sure you are exhausting the warm air away. and that cold air is coming into the tent. If you do this you will maintain an excellent temperature range.