Ideal setup for four fans is to have two blow in and two blow out. You are correct in your concern that the fans were NOT set up properly when you bought the used system. As you already suspected, all four blowing in will result in trapping much of the heat inside your canopy/lighting system, whereas having 2 blowing out the heated air will dissipate that heat into the room, thus cooling the tank (important in the summer time).
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