It'll stay above 140 degrees F, outside of the bacterial risk zone. Not only will this carry your brisket to dinner, I'd suspect it'll be one of the best ones you've made due to the very long rest. Don't open the cooler until you're ready to serve, though, you'll lose a LOT of heat every time it's cracked.
Personally, I plan for a minimum of 2 hours in a cooler, and don't even blink twice at 4 hours. 6 hours is not a problem.