For words that only have three letters, there isn't a 4th letter. One example is with the word “incorporate”, there are no a lot more letters Later on and so must be the word "increase". The word “addict” is inside the BIP39 wordlist however you would've utilised “addi” for “addict”. https://doctorbookmark.com/story18870783/bip39-seed-phrase-fundamentals-explained