For words that only have three letters, there isn't a 4th letter. For instance with the word “increase”, there aren't any extra letters afterwards and so have to be the word "add". The word “addict” is in the BIP39 wordlist however, you might have employed “addi” for “addict”. You could https://mediasocially.com/story4102553/not-known-factual-statements-about-bip39-explained