For words that only have three letters, there is no 4th letter. For instance with the word “add”, there won't be any much more letters Later on and therefore should be the word "insert". The word “addict” is within the BIP39 wordlist however, you would have utilized “addi” for “addict”. https://bookmarkcolumn.com/story18719161/the-greatest-guide-to-seed-phrase-bip39