Normally on the back side of the engine block, sometimes out of sight, there should be another ground running from the engine block to the body of the truck.
There are lots of good 4 ga wire brands out there. You generally want to use the same kind that you are using for your power wire, this ensures that you have the same strand count in both runs. Stinger makes some good wire, something in the HPM line. Knukonceptz makes good wire, I prefer them. I've heard that Kicker makes some decent wire, RF make some. As long as it's pure copper and has a lot of wire strands it should be good.