For words that only have three letters, there is absolutely no 4th letter. As an example with the word “insert”, there aren't any more letters afterwards and so has to be the word "add". The word “addict” is from the BIP39 wordlist but you might have used “addi” for “addict”. https://agendabookmarks.com/story18782539/fascination-about-bip39-english-words