I think that if I was a high dollar investor, I would be considering buying a copy of this book at this time.
As you state, demand for this book is at a low ebb. However, I don't think that that is a permanent situation.
Showcase 22 has been remarkably hot for the last few years, even though it is a much less important book than Showcase 4, and it is the first appearance of a second tier DC character as well.
I think interest in the Flash right now is lower than Green Lantern, but I don't think that is permanent. These things ebb and flow based on the quality of the modern books, movie interest, etc.
At some point I expect interest in the Flash to rise, and when it does I would expect Showcase 4 to rise in value compared to the other large keys from its current position.
Note-- all this said, I don't see myself ever owning this book. The only real high dollar books I am targeting in my collection are AF15 and ASM1. I may never own those either though.