For words that only have three letters, there is absolutely no 4th letter. Such as with the word “incorporate”, there won't be any extra letters afterwards and so should be the word "include". The word “addict” is in the BIP39 wordlist however, you might have used “addi” for “addict”. Mnemonic https://bookmarkrange.com/story20290916/considerations-to-know-about-bip39-word-list-pdf