Half windsor is fine as well, but sort of boring. The Four-in-hand gives a bit of character to the knot, if you can tie it right, you might need to practice whatever knot you choose to get the dimple right and length right, I wouldn't let it hang much lower than the bottom of your belt, a little higher would go better with the slim suit. And DON'T get black, just get Gray and change up the square, tie and shirt for Prom. Also, get proper laced dress shoes, no loafers and no squared toe shoes.
I'm expert at half windsors and I've got some burgundy ankle dress boots that ill try to match with the pocket square and knit tie.