For words that have only three letters, there is absolutely no 4th letter. One example is with the word “add”, there aren't any extra letters Later on and therefore should be the word "incorporate". The word “addict” is inside the BIP39 wordlist however you would have employed “addi” for “addict”. https://minibookmarking.com/story19587838/new-step-by-step-map-for-bip39-english-words