For words that only have three letters, there is no 4th letter. One example is with the word “include”, there isn't any far more letters afterwards and so has to be the word "add". The word “addict” is while in the BIP39 wordlist but you would've utilized “addi” for “addict”. https://onlybookmarkings.com/story18838001/the-invalid-bip39-phrase-diaries