For words that have only 3 letters, there's no 4th letter. One example is with the word “incorporate”, there aren't any more letters Later on and therefore has to be the word "increase". The word “addict” is within the BIP39 wordlist however, you might have employed “addi” for “addict”. A https://socialbuzzfeed.com/story4427177/not-known-factual-statements-about-bip39-explained