For words that have only three letters, there is absolutely no 4th letter. One example is with the word “incorporate”, there aren't any much more letters afterwards and so should be the word "add". The word “addict” is during the BIP39 wordlist however you would've made use of “addi” for https://bookmarkport.com/story21244522/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool