For words that only have three letters, there isn't any 4th letter. As an example with the word “insert”, there are no more letters afterwards and so needs to be the word "increase". The word “addict” is within the BIP39 wordlist but you might have utilised “addi” for “addict”. BIP39 https://tvsocialnews.com/story4450854/top-latest-five-bip39-words-urban-news