For words that have only three letters, there isn't a 4th letter. For instance with the word “insert”, there are no more letters afterwards and so have to be the word "add". The word “addict” is inside the BIP39 wordlist however, you would have utilized “addi” for “addict”. A seed https://webnowmedia.com/story4426915/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator