Tim isn't playing week 4 because week 4 is used for the backup QBs to practice. After being reminded of Timmy's lack of throwing ability last week, my guess is they're trying to let the real QBs get some reps.
Besides, with the role(s) he''ll be playing on O and ST, he's basically a starter anyways that should get some rest.
Or maybe they're already planning on sending him to Jacksonville for MJD. If that happens, Tanny is a genius again.