For words that only have three letters, there is not any 4th letter. Such as with the word “incorporate”, there won't be any extra letters afterwards and thus needs to be the word "incorporate". The word “addict” is during the BIP39 wordlist however, you would've used “addi” for “addict”. Mnemonic https://bookmarkmargin.com/story18806550/invalid-bip39-phrase-things-to-know-before-you-buy