For words that have only 3 letters, there's no 4th letter. By way of example with the word “include”, there won't be any more letters afterwards and thus has to be the word "insert". The word “addict” is during the BIP39 wordlist however , you would've used “addi” for “addict”. https://linkedbookmarker.com/story4988860/phrases-with-bit-an-overview