For words that only have 3 letters, there is not any 4th letter. One example is with the word “include”, there isn't any far more letters afterwards and so should be the word "increase". The word “addict” is inside the BIP39 wordlist but you would have utilised “addi” for “addict”. https://socialbuzzfeed.com/story4333695/5-simple-techniques-for-bip39-word-list-pdf