For words that only have three letters, there is no 4th letter. By way of example with the word “include”, there are no additional letters afterwards and thus must be the word "increase". The word “addict” is in the BIP39 wordlist however you would have utilized “addi” for “addict”. By https://easiestbookmarks.com/story19111611/considerations-to-know-about-bip39-word-list-pdf