For words that have only 3 letters, there is absolutely no 4th letter. As an example with the word “add”, there are no more letters Later on and thus needs to be the word "insert". The word “addict” is from the BIP39 wordlist however , you would've utilised “addi” for https://maroonbookmarks.com/story18814889/a-secret-weapon-for-bip39-words-to-numbers