$ perl -i -ple '$_.="\n" if $. % 4 == 0; close ARGV if eof' -- FILES

##</code><code>##

{ # new scope for local
	local *ARGV; @ARGV = @files;
	local $^I = ""; # -i command line switch
	while (<>) {
		print $_;
		print "\n" if $. % 4 == 0;
		close ARGV if eof;
	}
}