For words that have only 3 letters, there's no 4th letter. One example is with the word “add”, there isn't any extra letters Later on and therefore have to be the word "increase". The word “addict” is during the BIP39 wordlist but you might have utilized “addi” for “addict”. - https://bookmarkpath.com/story18820937/not-known-factual-statements-about-bip39-explained