For words that have only 3 letters, there is absolutely no 4th letter. For instance with the word “incorporate”, there won't be any far more letters afterwards and so needs to be the word "incorporate". The word “addict” is in the BIP39 wordlist however, you would've utilized “addi” for “addict”. https://bookmarkspy.com/story20182125/bip39-seed-phrase-fundamentals-explained