perl -e'$a="aaaa";until($a gt"dddd"){print "$a\n" if $a=~/[a-d]{4}/; $a++}' 

perl -e 'for $a ('a'..'d'){for $b ('a'..'d'){for $c ('a'..'d'){for $d ('a'..'d'){print "$a$b$c$d\n"}}}}'