For words that have only three letters, there isn't any 4th letter. Such as with the word “incorporate”, there isn't any extra letters afterwards and so have to be the word "add". The word “addict” is while in the BIP39 wordlist however, you would have utilised “addi” for “addict”. Mnemonic https://bookmarkstumble.com/story20504619/the-greatest-guide-to-seed-phrase-bip39