For words that only have 3 letters, there's no 4th letter. One example is with the word “add”, there aren't any additional letters Later on and therefore must be the word "increase". The word “addict” is inside the BIP39 wordlist however, you would have applied “addi” for “addict”. A seed https://yesbookmarks.com/story18965339/bip39-seed-phrase-fundamentals-explained