For words that only have three letters, there is not any 4th letter. As an example with the word “include”, there are no more letters afterwards and thus must be the word "increase". The word “addict” is from the BIP39 wordlist but you would've applied “addi” for “addict”. By following https://health-lists.com/story19666152/considerations-to-know-about-bip39-word-list-pdf