For words that only have 3 letters, there is not any 4th letter. One example is with the word “add”, there aren't any a lot more letters Later on and so have to be the word "include". The word “addict” is during the BIP39 wordlist however , you might have https://reallivesocial.com/story4340216/an-unbiased-view-of-bip39-seed-phrase-github