Not bad, but why try to match the tag again when you already let the engine match it before?

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

my $c=0;
s( (<span>|</span>|(.)) ) {
    defined $2 ? $1 . (++$c % 4 ? '' : "\n") : $1;
}sxge;

print;
print "\n";
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Makeshifts last the longest.