I seriously think you're already a really good artist and have a great foundation. Here's what I think about these specific pieces.
.
1 and 2 are lacking depth in the hair, but 3 and 4 look great! Ngl I'm jealous of how you do hair, I find you make it look so realistic, especially the way you do tails!
.
2 is my personal favourite, but you're missing the shadow under the hair. And for the others it a bit too black/unsaturated, so I'd try to use a darker version of a colour on an Overlay or Soft Light blending layer
.
The outline around your horses is a bit strong in my opinion, and I would consider lowering the opacity. (I always do that glowy outline thing because I find it to be the "secret sauce" of art, I just have the opacity quite low/blend it out more). Of course, this depends on what you prefer to do stylistically and what the lighting of the background is like!
.
I'm the 1st piece, I notice a bit of discontinuity in hair length. You have one really long strand in the mane that you can see beneath the horse, but then the rest of the mane doesn't seem to fall to the same length.
.
I find your eyes a little small, but I've heard that's actually because when we look at things in real life, we perceive the eyes as much larger because they hold more of our attention, so in art we tend to over exaggerate it.
.
Idk how to describe it, but your hair, especially the forelock, is "flat" in a way and could use some more "bounce". I honestly don't know how to get it there though.
.
In the 4th, there's a sort of lighting discontinuity, where the background is weirdly dark, and the horse seems lighter than the sky. The whites also tinged a bit too blue/green in my opinion, I would try making it warmer to make it pop.
.
I really like the light/backlit outline you did on the draft in 3. I would keep adding that to your pieces, especially 2!
.
Add a highlight to the eye to make it pop!
.
Try to include both warm and cool tones to your pieces, regardless of what the overall colour scheme is. I think that's why 4 ended up looking a little flat.
.
Make sure to always add variation to your blacks, bays and every other horse, in terms of saturation (I like to do the back darker, and the armpits lighter) and hue (for bays, yellower in the armpits and greater on the back).
.
I find you use a bit of a harsher white light to light your pieces, that's very dense. I would try putting that on overlay, softlight, or add so that it laters over the horses coat colour instead of it getting a "foggy" look.
.
Never use pure black or white! Always a darker or lighter hue. Can be almost that colour, but should still have a little hue in it.
.
Sorry if that's too much lol