yet dummy, as I could not execute the test myself.
Test data (ws281x_RGBW_4mhz_snippet.sr) is already available.
protocol-decoder rgb_led_ws281x channel din=0
input led/ws2812b_neopixel24/ws2812b_neopixel24_4mhz_snippet.sr
output rgb_led_ws281x annotation match ws2812b_neopixel24_4mhz_snippet.output
+
+test ws281x_rgbw_4mhz_snippet
+ protocol-decoder rgb_led_ws281x channel din=0 option type=RGBW
+ input led/ws281x_rgbw/ws281x_RGBW_4mhz_snippet.sr
+ output rgb_led_ws281x annotation match ws281x_RGBW_4mhz_snippet.output