Well - 4-12, 4-12, 8-8 is a total 16-32 record. That's a .333 win percentage.
That said, things like this should take into account team strength and whatnot. I mean if you had the Colts who went 1-15...but lost Peyton Manning, you SHOULD suck and if you do, it should be expected.
I do agree that 4-game improvement to respectability (heck, 8-8 has won divisions before, let alone playoff spots) should keep you on the "hot seat" but not necessarily outright fire you.
Does M13 have owner personalities? I know in OOTP there's stuff like Demanding, Tolerant, etc, that impact both expectations and how patient the owner is with you before firing you. It is silly you have to RETIRE - why can't your coach just look for another job. Not like we've never seen coaches change teams after getting the axe in the NFL...