Using awk you can split each line of a file into words and each word into single characters. If and only if a character in a word does not match the complement of abcd, the word is deleted:
for ( i = 1; i <= NF; i++ )
for ( j = 1; j <= length($i); j++ )
if ( substr($i,j,1) !~ /[^abcd]/ )
I'm not really sure this is what you're looking for, anyway!