I feel like this game has more "registering the key" kinda problems than the other one. There's also a kind of "artificial difficulty" that comes with songs like "I Just Wanna Burn" on pro where you just hit two keys repeatedly for the whole song pretty much. As good as this game is, it's not perfect and I hope some of its problems are rectified in the next one ;)
Guys, for 4th boss you have to play 3 purple swirled songs and get three dark orbs at the bottom right of the screen.Then play a 4th purple swirled song and itll trigger the boss fight.Hope it helps