Back in the days where there was no neopixel module we did this with send4bit(). I had no problems with RGBW led pixels. The pixels are SK6812 with three variants RGBNW (natural white), RGBCW (cold white) and RGBWW (warm white). I used them for a bike lights project. If you add plastic lenses to the chips, they get quite bright ;-)
Edit: there's a RGBW neopixel library at Adafruit.