For words that have only 3 letters, there is no 4th letter. One example is with the word “add”, there aren't any a lot more letters afterwards and therefore should be the word "increase". The word “addict” is while in the BIP39 wordlist however you would've employed “addi” for “addict”. https://www.anobii.com/en/01d109953642f9cd99/profile/activity