For words that have only 3 letters, there is not any 4th letter. As an example with the word “incorporate”, there are no far more letters afterwards and therefore needs to be the word "include". The word “addict” is within the BIP39 wordlist however, you would've employed “addi” for “addict”. https://gogogobookmarks.com/story19607094/bip39-seed-phrase-mnemonics-generator-an-overview