It looks a bit grainy, but that's probably the computer. I really like the concept though. I think you should cut out the excess black on the left side. Er...no wait, it's a wallpaper. Actually then, the black on the left is great. leaves space for icons while your still able to see Samus. I'm still sticking with 4.5 because of the graininess though.
I agree with Chinchillax, but minus extra points for the spark in Samus' beam cannon... It's way too bright to resemble a cannon charge, and if that was your goal, you could have manipulated it to look like it was partially inside the cannon hole. Also, the coloring job is a little on the deficient side, but I won't take points for that, since I couldn't do CGing even if my life depended on it.
Overall, and noticing the points already stated by my pal Chinchillax,