1) Sorry, this does nothing for me. With a tight enough crop (to remove the trees) and increased tonal range it might make a decent abstract as misstwinklytoes mentioned (you could even play with false color rather than sticking with B&W), but there's nothing really appealing to me about a jumble of concrete and rebar.
2) I really like this shot. The composition is great - I like how the perspective of the wall and the shadows create leading lines drawing the viewer right to the subject. It's a great tone of light, too. I don't even mind that the focus does seem to be on the foreground rather than the subject - having him slightly out of focus in this case adds to the mystery.
3) I agree that this is underexposed and would greatly benefit from some post processing. I think it would make a very cool abstract if you get rid of the dirt in the upper left, through some combination of cropping, cloning, distorting, etc.
4) Very nice! I like the composition. It's unusual to place the center of the flower so close to the corner but this is a great example of when violating the rule of thirds works. The only (minor) quibble I have with it is that the most-lit area seems to me to be in the upper right corner rather than the center of the flower, so there is some slight competition for where the eye is drawn.
Edit: Just saw the color version of #4. It looks like you nailed your lighting, the B&W version only seems dimmer in the center due to the natural coloring of the flower. I withdraw the quibble, Your Honor.