add some space to the top and bottom of the canvas, so that the characters head and feet aren't touching the canvas edge. Having them touch decreases the sense of space as it emphasizes the reality that the character is inside a flat box. Having objects edges line up exactly with the canvas edge also decreases the idea in the viewers mind that the world inside the canvas expands beyond its edges.

the face in the lower right look alot more like a goat now. Try to think minimally with logos. Remove the details that arent neccesary.

I think the red parts here arent added anything really except making it a bit noisier. Its good youre doing alot of variations on this design, but have you tried other designs as well?

one thing that improves a 4 frame cycle is not reusing frame 2 as frame 4. For the up and down directions, instead of using a standing pose for these frames, draw an actual inbetween frame for frame 2, it can be mirrored for frame 4. Take a look at this basic walk cycle example from the animators survival kit:

http://danielanimations.weebly.com/uploads/1/3/7/4/13748783/2610219_orig.jpg

For a 4 frame cycle we're mostly interested in the ones labeled contact and passing position. Youve already drawn the contact positions for frames 1 and 3, but it would look more proper to draw the passing position, with one foot lifted and moving forward for frames 2 and 4. Since the example is in side view, it should be pretty easy to follow for the left and right animations. Basically, you need your guy to pick his feet up.

I'd like to ask if there's a quick and easy way to find the exact color that lies in between the two other colors. Should the AA consider both hue, saturation and brightness?

Ya it should consider all 3. I just chose a color from your existing palette which seemed about in the middle, but if I were to add a color to the palette for the AA I'd approach it by thinking about the hue, saturation and brightness seperately first. Like in the example in my previous post, you want a color between black and a color fairly close to white, so you know right off the bat that the value will be close to 50%. IN this example the hue is easy, because you can just keep it the same, but if you were AAing between 2 very different colors you can pick a hue somewhere in the middle of those 2 colors or a gray. In such a case it would probably help to lower the saturation too, because when you mix 2 colors together (atleast in paints) the resulting color is less saturated. You probably dont want your AA to be too saturated in general, because otherwise it will stand out too much instead of blend in. I wouldn't worry too much about making the AA color perfectly inbetween though. Since youre doing it manually you can make it whatever color best suites your image and palette, you just need it to be close enough to be functional.

quick edit to try to put more detail in, even if its not the style youre going for