Sorry for the late reply, and thank you so much for your help.
high power PWM LED driver
Could you suggest me one? My first attempt will be to drive 4x 3W LEDs, each driven by a dedicated PWM signal. I reckon that TI AS1170 could do the trick but for two separate PWM-driven LEDS; could TI TPS92512 or TPS922055 be enough to drive the 4 LEDs?
some resistor
Would it be a good practice to add a high value (i.e., 10K) resistor between the negative pole of the supply and the GND of Arduino?