For words that have only three letters, there is absolutely no 4th letter. For example with the word “add”, there isn't any extra letters Later on and therefore has to be the word "include". The word “addict” is in the BIP39 wordlist however you might have utilized “addi” for “addict”. https://thebookmarkid.com/story19562235/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition