For words that only have three letters, there is no 4th letter. One example is with the word “insert”, there are no much more letters Later on and thus needs to be the word "add". The word “addict” is during the BIP39 wordlist however you would've utilized “addi” for “addict”. https://freebookmarkpost.com/story18726779/the-ultimate-guide-to-bip39-seed-phrase-github