For words that have only 3 letters, there's no 4th letter. For example with the word “add”, there aren't any much more letters afterwards and thus must be the word "insert". The word “addict” is in the BIP39 wordlist however , you would have used “addi” for “addict”. bip39-phrase.com is https://social-medialink.com/story4025096/an-unbiased-view-of-bip39-seed-phrase-github