For words that have only 3 letters, there isn't any 4th letter. As an example with the word “add”, there won't be any much more letters afterwards and therefore have to be the word "add". The word “addict” is while in the BIP39 wordlist but you might have employed “addi” https://socialdummies.com/story3789690/the-ultimate-guide-to-bip39-seed-phrase-github