For words that have only 3 letters, there isn't any 4th letter. By way of example with the word “include”, there won't be any more letters Later on and therefore has to be the word "include". The word “addict” is in the BIP39 wordlist however, you might have used “addi” https://bookmarkchamp.com/story19547646/the-ultimate-guide-to-bip39-seed-phrase-github