For words that have only three letters, there is absolutely no 4th letter. For instance with the word “incorporate”, there are no more letters afterwards and thus have to be the word "incorporate". The word “addict” is during the BIP39 wordlist however you would have made use of “addi” for http://andersonsaeu988.iamarrows.com/from-mnemonics-to-safety-and-security-recognizing-bip39-passphrases-and-their-significance-in-copyright