For words that only have 3 letters, there isn't any 4th letter. For instance with the word “add”, there isn't any more letters afterwards and thus must be the word "incorporate". The word “addict” is inside the BIP39 wordlist however you would've utilised “addi” for “addict”. This text points out https://bookmark-group.com/story4570980/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator