For words that only have three letters, there isn't any 4th letter. As an example with the word “increase”, there are no additional letters Later on and so must be the word "insert". The word “addict” is within the BIP39 wordlist however , you would have used “addi” for “addict”. https://bookmarkinginfo.com/story18817100/5-simple-techniques-for-bip39-word-list-pdf