For words that only have 3 letters, there is not any 4th letter. By way of example with the word “incorporate”, there are no additional letters afterwards and thus should be the word "add". The word “addict” is within the BIP39 wordlist however you might have used “addi” for “addict”. https://totalbookmarking.com/story18886640/the-2-minute-rule-for-bip39-all-words