For words that have only 3 letters, there isn't any 4th letter. By way of example with the word “include”, there aren't any additional letters Later on and therefore have to be the word "incorporate". The word “addict” is in the BIP39 wordlist but you might have made use of https://ilovebookmarking.com/story19604453/how-much-you-need-to-expect-you-ll-pay-for-a-good-valid-bip39-seed-phrase