For words that only have 3 letters, there is no 4th letter. For example with the word “add”, there won't be any much more letters Later on and thus must be the word "incorporate". The word “addict” is during the BIP39 wordlist however you would have utilised “addi” for “addict”. https://bookmarkedblog.com/story19902418/5-simple-techniques-for-bip39-word-list-pdf