For words that only have 3 letters, there isn't any 4th letter. One example is with the word “insert”, there won't be any a lot more letters afterwards and thus must be the word "incorporate". The word “addict” is inside the BIP39 wordlist but you would've utilised “addi” for “addict”. https://bookmarkoffire.com/story19393956/the-invalid-bip39-phrase-diaries