For words that have only 3 letters, there's no 4th letter. For example with the word “increase”, there aren't any a lot more letters afterwards and thus needs to be the word "insert". The word “addict” is while in the BIP39 wordlist however you might have utilized “addi” for “addict”. https://echobookmarks.com/story19012462/not-known-factual-statements-about-bip39-explained