Filled die error. I'm guessing it was probably a 1. Grease or gunk filled the numeral in the die and didn't strike the numeral. Not exactly rare but still neat.
Look at how fast these are minted. This is just an irregularity that happens from time to time. These machines run hot and that’s why they grease them, so they won’t seize up. That 4 caught a little of it in there, that’s why it didn’t strike fully.