For words that only have 3 letters, there isn't any 4th letter. As an example with the word “increase”, there aren't any more letters afterwards and therefore needs to be the word "insert". The word “addict” is while in the BIP39 wordlist but you would have employed “addi” for “addict”. https://myeasybookmarks.com/story4293099/not-known-factual-statements-about-bip39-explained