Controlling LED 5050 RGB Strip
The 4 conctact points are usually R/G/B anode/cathode and the 4th is common cathode/anode.
AFAIK both variations exist.

To control the brightness, the normal way is to use PWM - switch on the required power for a short period of time, switch it off. Do this at a high frequency.

By altering the time it is on/off, you can control the brightness.