I am an alley!
a
The letter that signifies the fourth sublevel is "f". It corresponds to the fourth energy level (n=4) in an electron configuration.
An owl signifies the letter m.
F
With an arrow signifying the direction of NORTH - possibly just with the letter N
(Vitamin) B2.
To get into the Forbidden Corridor on Pottermore, cast the Unlocking Charm (Alohomora) by either typing in the letters on the screen in the order the spell is read or by clicking on the letters when the line reaches the letter.
In football, the letter "H" typically signifies the position of a player as a halfback.
In guitar tabs, the letter "R" signifies a rest or a pause where no notes are played.
In linguistics or phonetics, a line over a letter signifies that the sound represented by that letter is pronounced with a higher pitch or tone.
In music notation, the letter "M" typically signifies a metronome marking, indicating the tempo or speed at which a piece of music should be played.
That's natural.