For words that have only three letters, there is no 4th letter. For example with the word “add”, there isn't any a lot more letters afterwards and so need to be the word "add". The word “addict” is during the BIP39 wordlist however you might have employed “addi” for “addict”. https://cyberbookmarking.com/story18953442/5-essential-elements-for-bip39-explained