Hmm...9 hours @ 180* on a 4lbr might be about right to bump it up later to bring internals up to finish temp when you get home, then foil and towel wrap to rest for an hour or so. Be sure to go to at least 180* internal temp to get it tender for slicing.
My only concern would be the lower smoke chamber temp for the duration...it may not pasteurize the surface/sub-surface of the meat properly. Typical low & slow temps are not under 225*, except for cured meats or smaller cuts which only take a few hours to cook through to finished temp. I definitely would not inject broth or marinade with this low of smoke chamber temp on that heavy of a cut of meat for food safety reasons (40-140*/4-hr guideline). I have started out low like that in the past, but always bumped temps up within an hour or so after start-up.
EDIT: btw, I'm assuming you're using a vertical electric (or possibly propane) smoker, I would also suggest using a dry water pan...toss in some washed pea gravel or sand for a thermal mass and use foil above the pea-gravel/sand as a liner fro drippings. This will produce a drier smoke chamber humidity which will seal in the natural moisture in the meat for reduced moisture loss on your lean-trimmed brisket flat...trust me on this, it works wonders on high finished temp lean meats.
Eric
