@1st Signature: BG's lacking depth and put more to it, it looks really dull/boring. The text is okay, nice placement for it. I suggest you also work on your lighting. Scanlines are okay, not bad, prolly not necessary though.

@2nd Signature: Left hand side looks flat to me. Work on your lighting. Text don't fit, work on the placement and colors, possibly font as well.

@3rd Signature: Blend the render in with the BG more, again, BG needs more depth. I think render's a bit too big, it's covering most of the signature's potential (Weeh, big words :P). My bad..And I don't know if the person who requested the signature wants a lot of text on or what, but, I don't think a signature should have too much text on, makes it look..euk. Text placement, font and color needs work.

@4th Signature: Blah, I think this is the worst out of all 4. More depth on BG needed and I really don't like that sudden change from a smooth abstracty feel to the grainy look. And text, again, needs work.

Overall, mainly focus on adding depth to your BG and prolly work on the lighting. Nice try though