Very cute game! Love the art and some fun platforming. There are a few items of feedback I would give though:
1) Guessing a 4 digit code at the beginning without any context can be really though and not super fun
2) Learning the game can be a lot of fun, but there are some things that aren't taught to the player in any organic way. The most extreme example of this is level 2. Without watching the youtube video, I would have tried to jump up on that platform over and over again without realizing the platform would just spawn back at the beginning.
3) There's nothing wrong with a hard game, in fact I love hard games. Especially hard platforming games. I actually think your difficulty is spot on for a challenging platformer, except for one thing. Restarting from the very beginning isn't actually a hard thing...it's just really frustrating and makes the player not want to repeat everything they've done already. Restarting a level over and over again has become something of a norm for hard platformers for a reason...the feeling of huge accomplishment happens every single level, not from beating the entire game in one go.
Anyway, I hope you like feedback! Those were my thoughts on the game, and overall I think it's really fun and that's why I have so much feedback because I would love to see it expanded on. Good job!