they are all beginnerish:
1st sig: its eh the white soft brush takes the focus away from the focal and the c4d is standing out a little bit from ur render
2nd sig: that black box in the sig is like wtf? lol so remove it u need to blend in the render more and make the text a little bit smaller imo and try to make the colors match more
3rd sig: take out the clipping mask cuz it looks bad and its distracting me move the text closer to the focal the render looks somewat pixeled towards the bottom of it and u should smudge the render more so it could blend in with the bg cuz it looks like there is a smudged or zoomed render under the render and it looks bad
4th sig: use a different render cuz it looks LQ and there r pixels on his hand and the text looks like its pixeled too and like the 3rd sig u should smudge the render to blend it more
other then all this keep practicing and follow tuts