8 gauge will do the job, if the battery is in the boot, but I would probably use 4 gauge for power and earth cables just to be safe, only if you do go the additional mono block amp. If its just the 4 channel amp 8 gauge will be sweet.
If you need to split the cables you can use a distributer block such as in the link:
http://images.google.com.au/images?sourcei...sa=N&tab=wi
This basically makes the cable go in as one 8 or 4 gauge and then spits out as many 8 gauge cables as you need (1 for each amp). These have fuses in them also, but you should still have a fuse near the battery.
You can use one 60 amp fuse, place closed the battery, the cable from the battery to the fuse shouldn't be longer then about 30cm (so Ive been told by many) otherwise it wount protect your equipment as well. If you start blowing the 60 amp fuses easily when you turn it up a bit, go for an 80 amp, and it should be all good. This will all depend on how much power your amps have.
Just make sure you have really good grounding points. This can make or break your system!