For words that only have three letters, there isn't a 4th letter. As an example with the word “include”, there are no additional letters afterwards and so has to be the word "include". The word “addict” is within the BIP39 wordlist however , you would've employed “addi” for “addict”. BIP39 https://socialdosa.com/story8660042/considerations-to-know-about-bip39-seed-phrase-generator