For words that have only three letters, there isn't any 4th letter. Such as with the word “incorporate”, there isn't any more letters afterwards and so should be the word "add". The word “addict” is during the BIP39 wordlist however, you would have used “addi” for “addict”. You ought to https://redhotbookmarks.com/story19560470/a-secret-weapon-for-bip39-words-to-numbers