Why loop (4*4*4*(23-2)) times when you can simply loop (23-2) times.

$_ = 'aaaabdbbdaccdacdaaabdcd';
++$counts{$1} while /(?=([abcd]{4}))/g;
print("$_: $counts{$_}\n")
   for sort keys %counts;
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If you want to see the "0" results, you can loop (4*4*4)+(23-2) times.

$_ = 'aaaabdbbdaccdacdaaabdcd';
++$counts{$1} while /(?=([abcd]{4}))/g;
print("$_: $counts{$_}\n")
   for glob "{a,b,c,d}{a,b,c,d}{a,b,c,d}";
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