For words that have only three letters, there's no 4th letter. One example is with the word “incorporate”, there isn't any much more letters afterwards and so have to be the word "incorporate". The word “addict” is from the BIP39 wordlist however , you would have applied “addi” for “addict”. https://bookmarksystem.com/story18657709/bip39-phrase-generator-can-be-fun-for-anyone