When you say random letters, I take that to mean you're getting lots of output outside of the 4 values in the array which gave me an idea. Does having
Keyboard.begin();in setup() make a difference? I don't have a Teensy nor Leonardo to test this with, so I created a small sketch that calls Serial.print without a Serial.begin call and found it compiled but didn't produce output either.
Stabbing blindly in the dark and hoping for the best,
Geoff