Looks good, I use a different PS than the M4 but I assume it has a delayed output (12v) like mine. If so, this is a good place to trigger your amp if you do not want to use a toggle switch.
Minor point, but if you do trigger the amp straight from the battery, don't forget to fuse that line nice & close to the battery just like your main 40amp line.
I'm not sure how you came up with 40amps for the main feed because I don't know your amp but assuming you've done the math and it is correct; 8ga. wire is only going to be good up to about 10 feet of wire then it will be too light for the job. Even if it squeaks by in theory, you might be better off to use 4ga. right out of the gate to eliminate that as a possible problem when the system is done, also it will allow for future upgrades should you decide to upgrade the amp or add a second one.
BTW, welcome to MP3car.com!