For words that have only 3 letters, there is not any 4th letter. Such as with the word “incorporate”, there are no additional letters afterwards and therefore need to be the word "incorporate". The word “addict” is within the BIP39 wordlist however , you would have used “addi” for “addict”. https://ztndz.com/story21971009/bip39-phrase-generator-can-be-fun-for-anyone