About those old 09s and 15s, they do fine on amazingly big props. Peter Chinn found both the 09 and 15 III would run and idle well on a TF 10 x 3 1/2 prop. As a practical matter, the 15 III and IV run great on a 9x4, and the 09 III and IV do well on an 8x4.
The 15 IV is heavier than the 15 III; I assume, as someone said, it's a little more powerful, but I have not done a comparison.
The 19 IV also handles large props very well, "diesel-like" as someone said. I've only tried a 10x4 and it handled that just fine. But my 19 V ran poorly on a 10x4 and was not happy till I put on a 9x4. That seems like the right prop for that engine. I assume the same is true of the later 19s.
Jim