For words that only have 3 letters, there is absolutely no 4th letter. For instance with the word “insert”, there won't be any far more letters afterwards and thus should be the word "insert". The word “addict” is within the BIP39 wordlist but you might have utilized “addi” for “addict”. https://travialist.com/story9718838/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting