For words that only have 3 letters, there is absolutely no 4th letter. For example with the word “insert”, there aren't any far more letters afterwards and so have to be the word "add". The word “addict” is within the BIP39 wordlist however, you would've utilised “addi” for “addict”. In https://extrabookmarking.com/story18968374/the-invalid-bip39-phrase-diaries