For words that have only three letters, there is absolutely no 4th letter. One example is with the word “add”, there isn't any a lot more letters Later on and so should be the word "include". The word “addict” is within the BIP39 wordlist however, you might have used “addi” https://meshbookmarks.com/story19085542/the-5-second-trick-for-ledger-seed-phrase-bip39