Show HN: Ray traced 4D Julia fractal renderer (Linux/Windows)

> The Julia set is a 4D fractal, that is defined as the points where the z(n) = z(n-1) + c series is divergent. As n and c are both complex numbers

Sorry, basic question: I'm only familiar with 2D – and, much less, 3D – fractals. How does that formula work? The n is usually the step number in a process, n=[0,]1,2,3.. I can't make much sense of using a complex number for that.

p.s. It should be z(n-1)^2 I guess in the formula, not z(n-1), no? (Although with complex n, who knows..)