Typical stereo headphones have 3 pins. Left, right, common ground. Tip, ring, and sleeve (not sure if the conductor order).

4-conductors used to be common for portable camcorders and early digital cameras. They’d put our composite a/v (extra conductor for video/yellow, still a shared ground). Tip, R1, R2, sleeve.

I’ve seen USB 2.0 (or perhaps 1.x) done over a 4-pin 3.5. And I’ve seen RS232 over 3.5 a number of times too (used to be common in ham radio in the 90s/early naughts).