For words that have only 3 letters, there is not any 4th letter. For instance with the word “increase”, there isn't any far more letters afterwards and so need to be the word "incorporate". The word “addict” is in the BIP39 wordlist however, you would've employed “addi” for “addict”. By https://bookmarkspedia.com/story4470073/5-essential-elements-for-bip39-explained