If you don't count first, then fourth is the third in line closest to zero. We're talking natural numbers, of course. Like, 2/3 of a diatom isn't quite a whole diatom. So forget fractions and negative numbers.
 Not music
If we were talking music, we would discuss the relationship of the fourth to other notes. If a clavier is involved, it had better be well tempered. Otherwise, we're back in the caves, hitting things with other things rhythmically.
At Harvard, they said, "you can't fish without bait." My uncle the fisherman knew better; he set to loaded with grenades.