The chances of finding a QB in the fourth round are slim but there's still a chance. They have shot themselves in the foot by not addressing the C and LB position, secondary depth is actually not that bad as long as they don't bring Nick Scott back.
Unless they make more moves in FA, I expect this to be an O-line heavy draft in at least the first three rounds. You can't go into the next season without a starting caliber LT and C.