For words that only have three letters, there isn't a 4th letter. One example is with the word “incorporate”, there are no more letters afterwards and therefore should be the word "add". The word “addict” is within the BIP39 wordlist however you would have made use of “addi” for “addict”. https://thesocialintro.com/story4395139/how-much-you-need-to-expect-you-ll-pay-for-a-good-valid-bip39-seed-phrase