For words that only have 3 letters, there is absolutely no 4th letter. For instance with the word “insert”, there aren't any more letters afterwards and thus have to be the word "insert". The word “addict” is in the BIP39 wordlist but you might have used “addi” for “addict”. Try https://socialbookmarkgs.com/story19191601/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool