For words that only have three letters, there's no 4th letter. For example with the word “insert”, there aren't any more letters afterwards and therefore should be the word "insert". The word “addict” is inside the BIP39 wordlist but you would've made use of “addi” for “addict”. Another benefit of https://wavesocialmedia.com/story4498256/fascination-about-invalid-bip39-phrase