For words that only have three letters, there is absolutely no 4th letter. As an example with the word “incorporate”, there are no much more letters afterwards and therefore should be the word "insert". The word “addict” is inside the BIP39 wordlist however , you would've employed “addi” for “addict”. https://baidubookmark.com/story18701873/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool