For words that have only 3 letters, there is no 4th letter. One example is with the word “add”, there isn't any a lot more letters Later on and thus must be the word "incorporate". The word “addict” is in the BIP39 wordlist however, you would've utilised “addi” for “addict”. https://trackbookmark.com/story20263619/phrases-with-bit-an-overview