although it's granular, i would add "except king" to the proposed sidegrade behemoth option. otherwise you get another solomater much like hauntedarmor - except without the ability to shred the "armor" prematurely.
I'm perfectly fine with greed not being able to fight behemoth though - serves you right for running him!
Also, for reference, the only cost-4 champions with attacks are envy, siren, thundermage and toad - so not a lot of "up" to this sidegrade, which I suppose is the point.

Some other suggested behemoth reworks I remember from the kong forum days include converting the full-blown immunity to a form of armor: either a "once-and-dunce" take 1 hit from any minion, or a moonfox-style "reduce value instead of dying" clause.