I dont really like how you've used a grunge style with the cars plus the black show throught the pixel strech too much. the 3rd is proberley the best but lower the contrast and make the text more readable/visable. On the first and fourth the renders both look lightly blurred which spoils them a bit also add a boader to all of them
srry if this seems a bit harsh
p.s. get some new brushes youve used the same one in all of the four sigs which kinda makes them all look samey
there not bad but keep practacing and stick with your current
