For words that only have three letters, there is not any 4th letter. One example is with the word “increase”, there aren't any extra letters afterwards and therefore must be the word "incorporate". The word “addict” is from the BIP39 wordlist however you would have applied “addi” for “addict”. - https://wiishlist.com/story19421285/the-definitive-guide-to-bip39-seed-phrase