For words that have only 3 letters, there's no 4th letter. One example is with the word “insert”, there isn't any far more letters Later on and so must be the word "increase". The word “addict” is in the BIP39 wordlist however , you would've utilized “addi” for “addict”. This https://tetrabookmarks.com/story19637386/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition