What four bosses are on the Difficult Boss Battles Mount Rushmore?

Board 8

The official song of the Hot Sandwich Mount Rushmore , which is a "funky rap" for some reason:

https://suno.com/song/12981719-77f7-428d-acd5-083960a463c8
(|| ' ' ||) When have you ever seen three big, very smooth arcs
. /|_|\ of hair sticking out of a person's forehead? I haven't!