Does anyone know how many of the 4" nickel with magna grips and humpback hammer were made so I can know how hard to find they are?
With complete access to factory records that question is probably answerable, but it would take a lot of time.
At the level of approximation, we can observe:
About 11,000 HDs total were manufactured before WWII.
Maybe half of these were produced during the time when magna stocks were an available option.
Let's say nickel guns were half of all production, though I have no idea if that is true or not.
Maybe a quarter of all HDs were produced with 4" barrels.
The humpback hammer was an available option for the last couple of years of production, but I doubt it went on more than 10 percent of the HDs sold in that period.
You can't just multiply the fractions together to get a number, because those are not independent variables. But on a "feels-like" basis, I'd have to say it feels like there are probably not more than a couple of hundred guns at most that were shipped from the factory in the configuration you describe, if even that many. There might not be more than a couple of dozen. A complicating issue is that both magna stocks and humpback hammers could be separately purchased, so anybody who owned a Heavy Duty with service stocks and a standard hammer could dress his gun up after the fact. You couldn't tell a gun with later additions from a factory original without a factory letter.
In any case, the answer to your question is, "Not too many."