For words that have only three letters, there's no 4th letter. One example is with the word “increase”, there isn't any much more letters afterwards and thus has to be the word "add". The word “addict” is while in the BIP39 wordlist however, you might have utilized “addi” for “addict”. https://bookmarkloves.com/story20852554/5-simple-techniques-for-bip39-word-list-pdf