Yet another way to skin this cat (sorry for extending ww's animal metaphors...).

When I hear "count characters", grep in scalar context is usually the first thing to enter my brain.

use warnings;
use strict;

while (<DATA>) {
    print if ((grep /r/, split //) == 4);
}

__DATA__
r 3 r r
rr
4 r r r r
nada nothing zilch
to many rr rrrr rrr 's
[download]

Not sure how this compares to others' regex and tr solution's from a performance standpoint (and I'm too Lazy to benchmark it :)