For words that have only three letters, there isn't any 4th letter. One example is with the word “increase”, there isn't any extra letters Later on and so has to be the word "insert". The word “addict” is from the BIP39 wordlist however, you would've utilised “addi” for “addict”. The https://siambookmark.com/story18938764/top-guidelines-of-bip39-word-list-pdf