For words that have only three letters, there is absolutely no 4th letter. By way of example with the word “insert”, there are no far more letters afterwards and thus must be the word "insert". The word “addict” is from the BIP39 wordlist but you would've utilized “addi” for “addict”. https://sites2000.com/story8682105/facts-about-bip39-seed-phrase-github-revealed