For words that have only 3 letters, there is absolutely no 4th letter. As an example with the word “include”, there aren't any more letters Later on and thus has to be the word "add". The word “addict” is during the BIP39 wordlist however you would've utilized “addi” for “addict”. https://infopagex.com/story4194729/5-simple-techniques-for-bip39-seed-phrase