For words that only have three letters, there's no 4th letter. By way of example with the word “add”, there isn't any additional letters afterwards and thus need to be the word "add". The word “addict” is in the BIP39 wordlist however , you might have used “addi” for “addict”. https://bookmarkingquest.com/story18786491/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition