[zdw]

I was thinking the same thing until I looked at Perl, which has 4 being quite frequent.

The reason, of course, is that 4 is also $, which is used to denote a scalar in Perl.

Thus, because 5,6,7 correspond to %,^,&, which generally get used to a lesser degree for things like modulo, hashes, exponentiation and logical-and, they're used less.