For words that have only three letters, there is absolutely no 4th letter. One example is with the word “incorporate”, there are no additional letters afterwards and therefore need to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however you might have applied “addi” for “addict”. https://bookmarkedblog.com/story19519909/5-simple-techniques-for-bip39-seed-phrase