Thanks for the feedback! I understand that level 4 is really difficult, and even with the hint, not everyone got through it. Regarding fullscreen mode, I'll definitely try to fix it in the updated version of the game!
Thank you! I'm glad you liked it! Levels 4, 5, and 6 are passable, but they're very difficult and confusing. I should have added more hints besides the one in the center of the screen on level 4, or redone all of these levels.