Two easy, not-slick ways:

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"}}}}'
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