Excellent shots! Here are my thoughts on them.
1. Something that looks a bit off to me in this photo is the water... Looks like it was tilted, giving you a sort of diagonal horizon. Since the pillars of the bridge are perfectly vertical though, the camera wasn't tilted and it's probably just the bridge covering up some of the water. As is suggested above, I would recommend shooting this scene earlier in the night with some light left so you can get some more detail in the area where the water meets the bridge.
2. I love the vibrant colors on this one! Is it HDR? The only thing I can possibly recommend is to rotate the camera slightly to the left, so the passageway is on the right side of the photo and we can see more of the art on the wall. That way you get the contrast between the beautiful graffiti and the dank, grungy area in which it's located.
3. This one is visually very interesting, however I agree with the others that it looks a little too HDRed. Letting the shadows stay a little darker might improve the over-all effect of this image.
4. My favorite of the bunch! It looks so serene. I would suggest cropping (or re-shooting if you can) so that more of the sky is shown than the grass. This is because the sky is more interesting than the grass and changes in appearance the higher it gets, so the eye wants to see more of it. The grass on the other hand, though it has an attractive texture, is relatively monotonous so we don't need as much of it. The soft, warm lighting on both the horses and the barn is perfectly captured.