For words that have only 3 letters, there is no 4th letter. One example is with the word “increase”, there aren't any additional letters Later on and therefore have to be the word "incorporate". The word “addict” is within the BIP39 wordlist however you would've employed “addi” for “addict”. The https://sound-social.com/story8802984/little-known-facts-about-bip39-words