For words that have only three letters, there is no 4th letter. As an example with the word “insert”, there aren't any more letters Later on and therefore has to be the word "incorporate". The word “addict” is while in the BIP39 wordlist however , you would have utilized “addi” https://bookmarksurl.com/story4343080/getting-my-bip39-words-to-numbers-to-work