For words that only have 3 letters, there is no 4th letter. Such as with the word “incorporate”, there are no extra letters afterwards and so has to be the word "add". The word “addict” is while in the BIP39 wordlist however, you would have utilised “addi” for “addict”. Several https://fellowfavorite.com/story19959087/invalid-bip39-phrase-things-to-know-before-you-buy