For words that have only 3 letters, there isn't any 4th letter. For instance with the word “increase”, there aren't any additional letters afterwards and so needs to be the word "increase". The word “addict” is from the BIP39 wordlist but you would've applied “addi” for “addict”. BIP39 and forty https://linkingbookmark.com/story19368707/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting