For words that have only 3 letters, there is absolutely no 4th letter. Such as with the word “incorporate”, there aren't any extra letters afterwards and so have to be the word "incorporate". The word “addict” is in the BIP39 wordlist but you would have made use of “addi” for https://userbookmark.com/story18772217/a-secret-weapon-for-bip39-words-to-numbers