For words that have only 3 letters, there isn't any 4th letter. For instance with the word “increase”, there aren't any additional letters afterwards and thus must be the word "insert". The word “addict” is while in the BIP39 wordlist however you would have made use of “addi” for “addict”. https://bookmarkplaces.com/story19412282/5-simple-techniques-for-bip39-word-list-pdf