For words that have only 3 letters, there isn't any 4th letter. One example is with the word “include”, there isn't any additional letters afterwards and so should be the word "insert". The word “addict” is while in the BIP39 wordlist but you might have used “addi” for “addict”. The https://pr1bookmarks.com/story19062798/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition