For words that only have three letters, there is absolutely no 4th letter. For instance with the word “increase”, there aren't any extra letters afterwards and so need to be the word "include". The word “addict” is in the BIP39 wordlist but you would have utilised “addi” for “addict”. bip39-phrase.com https://cyberbookmarking.com/story18834514/the-2-minute-rule-for-bip39-all-words