07-31-2021, 07:53 AM 
	
	
	
		Try the gmic plugin http://www.gmic.eu  colormap filter. Adaptive mode - dithering zero - tones 4
 
   
At least it keeps the image in RGB mode, retains some semblance to the original (98000 colours) and does actually result in 4 colours (as set in gmic)
As with all graphics, a lot depends on the original image.
	
	
	
At least it keeps the image in RGB mode, retains some semblance to the original (98000 colours) and does actually result in 4 colours (as set in gmic)
As with all graphics, a lot depends on the original image.

 

 
