the bottom 2 are drawn better, but also the bottom 2 are framed in the canvas much much better.
I might scrap the top 2 not only because the drawing errors are much more noticeable, but also because they are pretty weakly framed. The top left seems too far away and a weak pose.  The top right has a nicer pose and angle and immediately looks stronger, but the composition is still much weaker than the bottom too how the top is cut off and the chin is too close to the edge. 

That out of the way, I think the bottom two are actually done pretty well!! Stylish, sexy.  The biggest problem I have is with the colors.  This is where the bottom two look most amateurish.  The actual potential behind the nice drawing/cool hair and so forth are masked behind poor colors that fail to really show off how well you did.  It is like if a master artist draws a masterpiece that looks basically photorealistic and then takes all the colors and makes them nearly the same.  It'll look very washed out, flat and maybe even amateurish, even if you could see the immediate genius behind the lines and detail etc.




Changing your colors was really only a 3 step process.



1.) I upped contrast so it looked less washed out.  I usually do this automatically with contrast slider.  Upping contrast just made the colors pop out more from the canvas. 

2.) Then once the colors were less ghostly, I had to gamma correct.   https://en.wikipedia.org/wiki/Gamma_correction  Basically I manually adjusted how dark shadow colors were, how mid mid tones were, and how bright highlights were individually through out the whole palette, as well as thinking about how these changes should mirror the surfaces the colors apply to most.  EG, gamma adjusting the skintones is different than for the hair.  The skin colors should be flatter and more claylike- colors smoothly fade from light to shadow.  I imagined the hair of the women as pretty glossy, so this is where really bright highlights poking through the strands works.  Note, if the skin was supposed to look more wet or oily, or if the hair was less shiny, adjusting gamma for those areas would be different. 

3.) Once gamma was adjusted, I focused on changes to mostly hue, but also value here and there.  This last portion is to go for a better unified and harmonious palette and to make sure all of the hue play within the image looks as tight as can be.  For example, I made the mid hair color on the right more purple, so the highlight colors didn't clash.  But then I lightened the skin tone up in value some just because I thought it made more sense for her, and a brighter more whiter skin also seemed better to mesh with the interesting hues going on in the image, such as the orangy skin shadows and so forth.  for the woman on the right, I decided to make the skin warmer and darker because I felt it looked better with the high contrast green hair.  I also then made the hair warmer as well because my intuition said that it just fit the character more.  I find this step just takes your intuition.  There are obviously no rules saying I had to make the skin on the right whiter etc., but I just felt out this step of the process with intuition
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The actual pixeling technique COULD be tighter, less noise, better clusters and so forth.  IE, better control and attention, but it is entirely feasible in my opinion, and some things are pixeled quite well, such as the eyes in the bottom right.