How can I use for in if?

How can I get this working? for wont count up in if.

      if (data.tSwitch2 == 1) {
        for(int b=1;b<4;b++){
          unsigned long current_time=millis();
          if(current_time - timestamp > interval){
            Serial.println(b);
            leds[b].setRGB(255,255,0);  // Setze Scheinwerferfarbe HIER!!!
            leds[b].fadeLightBy(ledbrightness);
            timestamp = current_time;
            // don't forget FastLED.show(), to output the colors
          }
          FastLED.show();
            }
      }