Rush hour traffic in Atlanta is murder. It runs more or less smoothly until you hit the "Downtown Conector" where I-75 and I-85 meet, then all bets are off, even in the "Express" lanes. Unfortunately, there's no easy way to a MARTA station from Kennesaw, because MARTA is the best way to get through Atlanta at rush-hour. I'd leave earlier than 4:30 if it were me, because sometimes security at ATL can be slooow as well, even in the Medallion line.