No, if you are truly at 300°F, then check it at four hours. If you are at a grate temperature of 250°F, then check it around 7 hours. It will be that much different in amount of time. This all assumes you are not wrapping.
It likely won't be done at 7 hours, but, you will start to get a feel for what is happening.
For the record, I dislike using internal temperature or time, alone, for guidance. I tend to poke the meat with my fingers, but, that would require a lot of coaching, or lots of cooking. I feel that I can poke, or squeeze the brisket and get a good feel for where I am in a cook. But, that is ridiculous to try and teach
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
Whip It Off, Chambers!
"perhaps...but then again...maybe not..."