The legacy adapter is for the Xbox One, and allows you to use your Xbox 360 guitars on the Xbox One. I have a Xbox 360, and went PS4 this time around. The dongle we mentioned are specifically for the Ps3. Do the Ps3 wireless guitars work on the PS4 without the dongle? Probably not... time to google some more... Found the answer:
Edit: Answer is yes, you still would need that USB dongle that came with the PS3 instruments, which pretty much every single guitar on Ebay for PS3 does not come with. lol.