For words that only have 3 letters, there's no 4th letter. As an example with the word “insert”, there won't be any a lot more letters afterwards and thus needs to be the word "insert". The word “addict” is while in the BIP39 wordlist however you would have utilised “addi” https://totalbookmarking.com/story19025422/a-review-of-bip39-phrase-list