You can theoretically connect as many fans as you want. It simply has three inputs on the controller. You can control the fans connected to each input. So if you want to control your four fans separately (speed/RGB etc.), three inputs are not enough. If you want to control all four fans identically, one input would theoretically be enough. Accordingly, you must connect at least 2 of your fans to one of the inputs with a Y-switch.
EKWB EK-Cable Y-switch for 3x 4-pin PWM fans (10cm)