For words that only have 3 letters, there is no 4th letter. One example is with the word “insert”, there won't be any far more letters afterwards and thus have to be the word "increase". The word “addict” is from the BIP39 wordlist however, you would've made use of “addi” https://ok-social.com/story4975941/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting