For words that have only 3 letters, there's no 4th letter. One example is with the word “add”, there won't be any additional letters afterwards and therefore should be the word "include". The word “addict” is within the BIP39 wordlist however you would have employed “addi” for “addict”. A straightforward https://bookmark-group.com/story4412069/an-unbiased-view-of-bip39-seed-phrase-github