What ambrus says. However, what it won't do, is give you the names of permutations with no occurrence in the string.

If you need those, I think I can find a shortcut. Think of such a string as a 4 digit number, in base 4, with an a posteriori mapping of

( 0 => 'a', 1 => 'b', 2 => 'c', 3 => 'd' )
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or, in Perl code:

tr/0-3/a-d/
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so all you really need is a 4 digit base 4 counter and this mapping. If you don't want to code this by hand, you might make use of a module like the one I found using a quick Google search, Math::BaseArith.