For words that only have three letters, there isn't a 4th letter. For example with the word “insert”, there are no additional letters afterwards and therefore need to be the word "include". The word “addict” is in the BIP39 wordlist however , you would've utilised “addi” for “addict”. A seed https://bookmarkity.com/story18984641/the-greatest-guide-to-seed-phrase-bip39