OH!, iīve been meaning to reply to this adorable rendition of Keen, and you took all of my suggestions out of my keyboard!
Some sort of contrasting (either coloured o pure black) outline was missing from this in order for it to really pop out.

Even tho, i really like the color scheme you chose (it pays a real tribute to the original keen), since you already reworked a bit of the character into "chibi" form, you might want to delight us with a more modern pallette? (this is more of a suggestion th)

Some more crits;
-I see some issues with Keenīs left hand on the "standing" sprite tho, since it isnt very readable at the moment.
-The shading seems a bit messy at parts (that lonely purple pixel on the shirt is killing me!), specially in the animation, and most noticeable on the pants.
-Im aware that you limited yourself both with the size, and the frame number but;
                       a) - I really think you could to better than just "chop" the top of the helmet, you can lower his head a few pixels, with almost no lost;
                           - Of course, this would mean also lowering the standing sprite (assuming they are treated "as one") in order for it to match the height of the animation;
                           - I think that, that was a mistake on your part, since you should have contemplated that, while creating it, otherwise you could have been more creative with the animation, in a way that the sprite isnt chopped (for instance, he could be sort of hunched forward, with a more "power walk" and he would fit the 32x32 AND also be more "true" to the original source)
                       b) - The 2-frame movement of the torso seems really stiff, and kinda breaks the flow of the animation;
                           - You can try to make his helmet move a bit independently of the head (sort of bouncing down for instance), that would make him have more character, and break the stiffness of the movement (more independence);
                           - You have 4 frames, but you are only taking advantage of 3! make it count!, at the moment the leg movement seems confusing also, because a great part of it sort of "repeats" or "remains" the same during the animation (the same happens with his right shoulder for example);
                           - Try to make the shading change during the different frames; for instance, Keenīs entire right arm "leaves the torso area", and the shading doesnt reflect that.


Cheers! Keep at it, im loving it, and im surprised no one else replied!