Agreeing with ali_ath72 that the game seems  unusually difficult.  

It feels like the difficulties are actually one setting above what they say (i.e. easy is more like normal, normal is hard and hard is impossible).  I think adding in a very easy mode would be beneficial to get players used to the controls.  In order to still encourage players to go to harder difficulties you could either cap the scene progression at lower difficulties or add a special end scene at higher difficulties (assuming this isn't already done).

For people struggling, I did find that reducing the game speed to 3 helps a lot.  At default speed, the beats aren't on the screen long enough to really see what's coming.  At lowest speed, they bunch up together too much, making it difficult to tell the order.    

In-regards to simultaneous beat inputs, I would suggest making them different colors from normal beats.  That would make them much easier to tell them apart from rapid successive inputs, which become more common at higher difficulties.  Also, since people have 4 fingers, it would be fun to see instances where you had to press 3 or 4 inputs at once.   Dance rhythm games only stopped at 2 because people only have 2 legs.