For words that have only three letters, there is absolutely no 4th letter. One example is with the word “include”, there are no more letters Later on and so have to be the word "insert". The word “addict” is while in the BIP39 wordlist however , you would've utilised “addi” https://captainbookmark.com/story18940579/not-known-factual-statements-about-bip39-explained