OP, I heard the same thing applying this year. I completely agree with what TimesNewRoman said, but I'd add one more thing. You will never know the material for step 2 any better than immediately after third year. The knowledge will just deteriorate as you go through fourth year, requiring more study effort if you decide to wait. I also did well on step 1 but decided to forgo my school's advice and take the exam early. I improved my score without much effort. This not only made fourth year much easier, but I also saw classmates struggle and dread studying for it later on. My reasoning was: you're either a good test taker or you're not, and in this case you clearly are and therefore shouldn't have anything to worry about by taking it early.
Ultimately, like TNR said, there's no wrong answer. But from my perspective, fourth year was much better without any exams looming. And I do think programs liked that I wasn't trying to game the system and had nothing to hide.