In memory of "Master Of Magic" and with help of the SDK I created a 4th yield (besides food, production and commerce). And it worked
.
But it it's neither shown on the map nor in the city view. But it's there, the tooltip confirms it. Although it does nothing useful (yet).
A screenshot:

But it it's neither shown on the map nor in the city view. But it's there, the tooltip confirms it. Although it does nothing useful (yet).
A screenshot: