383 Magnum was rated at 5200 rpm.
If you couldn't get your 383 past 4000, there was something seriously wrong with it.
If you couldn't get a 383 four speed to roast tires better than a 318 2bbl, there was something SERIOUSLY wrong with it. (I would guess trash valve springs, or a flat camshaft).
Probably valve springs going away, it ran fine otherwise. I sold the car last year so I'll never know. Still not a fan of 383s, they just don't do it for me.