Maybe 7 is the magic number cause it divides only by 1 and itself. 10 divides by 1, 2, 5 and itself and 4 by 1, 2 and itself. Maybe the magic number is who divides for less possible numbers, I guess.
It's (7) called a "cousin number".
)Sorry my grammar cause I', brazilian I'm a beginner in the English language.) :]