For words that only have three letters, there is not any 4th letter. As an example with the word “insert”, there won't be any far more letters Later on and so must be the word "increase". The word “addict” is during the BIP39 wordlist however, you might have utilized “addi” https://pageoftoday.com/story4456482/not-known-factual-statements-about-bip39-explained