For words that only have three letters, there is not any 4th letter. Such as 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 within the BIP39 wordlist but you would've applied “addi” for “addict”. https://bookmark-dofollow.com/story22182681/not-known-facts-about-bip39-english-words