8 awg is sufficient for a 5 pill as long as the run isnt too long. Looks as though you're setting up a Suburban so the run might be longer than desired, in which case 4awg is the better choice. Either way 4 awg will not be ideal for anything much larger than an 8 pill. So if you have plans of upgrading to 8+ pills in the future, you may want to throw down a pipeline of 0 so you won't have to screw around with rewiring it later on; it also affords the luxury of not needing to upgrade when the Power Bug bites you in the ass...and it WILL bite you, make no mistake about it.

The fine stranded car audio-style wire is the best choice. Dont get roped into the rookie error of buying Walmart 4awg jumper cables and using that for power. Also don't splice any wires if you can avoid it. If you absolutely have no choice and have to, use the proper sized butt connector, strip off the plastic jacket, crimp it all together and SOLDER the connection. But again it's best not to splice if it can be avoided.

Good luck