For words that have only three letters, there is absolutely no 4th letter. Such as with the word “insert”, there aren't any extra letters afterwards and so need to be the word "incorporate". The word “addict” is from the BIP39 wordlist however you would've applied “addi” for “addict”. This informative https://bookmarkindexing.com/story18702593/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool