Really great work here! The idea is flawlessly executed, and for a short timeframe, it is very well balanced. The idea to just keep the player to 1-3 for the first part, building their confidence and teaching the mechanics, before getting the other 3 involved, was a really great hidden tutorial. You were also good, whether it was intentional or not, at keeping the fairness appropriate in terms of speed. I think the game slowed down when the first piece with a "5" on it started coming down.
I honestly have no feedback other than great work! One minor suggestion would be to allow the player to slide the tetris pieces along for the "last minute" reflex save. That felt a bit inconsistent to me, and it seemed that it wanted to snap my pieces back.
I also really liked the game moment when I realised that ALL touching blocks clear. I was trying to make tetronimos specifically. That opened up the game for me.