For words that have only three letters, there's no 4th letter. As an example with the word “increase”, there won't be any much more letters Later on and so needs to be the word "incorporate". The word “addict” is during the BIP39 wordlist however, you would've utilized “addi” for “addict”. https://olivebookmarks.com/story18919334/top-guidelines-of-bip39-word-list-pdf