For words that have only three letters, there isn't any 4th letter. As an example with the word “add”, there isn't any more letters Later on and thus need to be the word "insert". The word “addict” is in the BIP39 wordlist however, you would've employed “addi” for “addict”. By https://hyperbookmarks.com/story18945225/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator