For words that have only 3 letters, there is absolutely no 4th letter. As an example with the word “insert”, there isn't any more letters Later on and thus need to be the word "increase". The word “addict” is in the BIP39 wordlist but you would've utilised “addi” for “addict”. https://bookmarkwuzz.com/story18859407/the-2-minute-rule-for-bip39-all-words