I wouldn't rely on the fans to be the primary means of cooling.
You can also hook all 4 fans in series, and then hook those up in parallel with your LEDs, so that your fans will somewhat adjust their speed based on LED level. (4 12v fans in series = 1 48v load). Also alleviates having a 12V converter.
Not exactly sure how your lights are set up, but the more airflow you have over the water, the faster it will evaporate, so that might also be a factor, all 4 fans might cause you to have to fill up the tank with water much more often, if you don't have an auto top off or RO system or whathave you.