For words that only have three letters, there's no 4th letter. For example with the word “insert”, there won't be any additional letters afterwards and therefore have to be the word "include". The word “addict” is in the BIP39 wordlist however, you would have used “addi” for “addict”. Artwork is https://allyourbookmarks.com/story18915837/a-review-of-bip39