For words that only have 3 letters, there isn't any 4th letter. For example with the word “insert”, there aren't any additional letters afterwards and so has to be the word "add". The word “addict” is inside the BIP39 wordlist however , you would have utilised “addi” for “addict”. To https://getidealist.com/story20607667/bip39-seed-phrase-mnemonics-generator-an-overview