For words that have only three letters, there isn't a 4th letter. As an example with the word “increase”, there won't be any additional letters afterwards and so has to be the word "add". The word “addict” is from the BIP39 wordlist however you would've employed “addi” for “addict”. BIP39 https://yoursocialpeople.com/story4177473/top-guidelines-of-bip39-word-list-pdf