It’s not quite fisheye as it’s more cylindrical than spherical, but UE4 does now support Panini Projection out of the box thanks to the UT project (R.I.P.). Nothing is really required to enable it other than running the command on that page.
If you need something much more closer to a real fisheye, and you’re much smarter than I am, then someone has an opensource project on github called Blinky that adds a bunch of different projection methods to Quake. The author has also put together some very interesting visualizations about how projection works as well as done a presenation (part 1, part 2) on it with ANOTHER developer who seems to have livestreamed much of his effort implementing the effect in Super Mario 64. It’s also been partially implemented by someone in Godot.
Unfortunately it is very expensive, requiring rendering from anywhere from 3-6 different views depending on the projection mode you choose. (maybe 5 if you only want fisheye) and probably more unfortunately, there are noticeable seams between the projections due to certain effects, especially camera facing billboards and screenspace effects. Sadly I don’t know of any better way to accomplish this.