So, it seems obvious that if the extant three steps are all curvy, that maybe the missing one was, too?
But I don’t think so.
There’s the 1999 image of a straight run. And the steps might be original. They were rotted enough to be.
Thus, my current thought is a straight run. The exposed side would be infilled with lattice, matching the adjacent lattice. I have no idea what a railing would look like, although I do have the original rail for the south entrance steps. I might have it copied.
The whole would be made out of pressure-treated lumber so it will last till the end of time. I won’t paint it.