A gene on the Y chromosome triggers sex differentiation in XXXY individuals after five weeks of development.

“So, all right. You and I are probably the only ones at this point this excited about this discussion because we're now so far down the weeds of embryology. But, um, sort of bringing it back to the surface, the takeaway here is that XXXY start out for about 5 weeks indistinguishable. At about that fivew week mark, a gene on the Y chromosome specifically begins to trigger the differentiation pattern. That differentiation pattern triggers the transcription of hormones that then transcribe”