1. 4 awg to the trunk is fine unless you're pumping more than like 1000 watts.
2. I think the typical recommended stereo fuse is like 60 amps. That's what I use.
3. Ground wire shouldn't go back to battery, it should go to the chassis (which returns back to the battery). This should be the same size as the +12v (4 awg).
4. IGN wire can be very small. I use 18 awg.
5. I use a molex connector spliced to an outdoor 3-prong extension cord to run from my PC to my screen/DVD/USB hub. It carries 12v, 5v, and ground. I believe the wires are 16 awg.
Your diagram shows the screen power wires running down the center with the VGA and USB cables. Don't do this, as you'll get distortion on your screen from interference. I know this from experience.
Wiring a CarPC isn't much more complicated than a stereo. Just make sure you think it through before hand and have enough cables to go around. Hopefully you won't be pulling up your carpet as many times as I have. :-) IMO, doing it yourself is good because then you know how it's put together in case something goes wrong.