There was a question here about how one can justify that integration does indeed give the area under a curve. Specifically, the OP brought up this image. The OP was troubled that even though Riemann sums intuitively "should" give the area, staircase approximation like in that image "should" give the length, so they doubted their intuition.
I found a few questions that used that image, but none of them were the one I wanted, which was specifically about Riemann integration. I don't think the question title even mentioned pi = 4, it was framed as a question about integration.
These aren't it: