For words that only have three letters, there is no 4th letter. For instance with the word “insert”, there won't be any far more letters afterwards and so has to be the word "incorporate". The word “addict” is during the BIP39 wordlist however you might have applied “addi” for “addict”. https://bookmark-master.com/story18852909/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool