For words that only have three letters, there's no 4th letter. By way of example with the word “add”, there won't be any more letters afterwards and therefore should be the word "add". The word “addict” is while in the BIP39 wordlist but you might have applied “addi” for “addict”. https://bookmarkgenious.com/story19270272/5-simple-techniques-for-bip39-word-list-pdf