$_ = "foo<span>bar</span>bazqux<span>quux</span>";

my $c=0;
s( (<span>|</span>|.) ) {
    my $s = $1;
    $s =~ m!</?span>! ? $s : $s . (++$c % 4 ? '' : "\n");
}sxge;

print;
print "\n";