For words that only have 3 letters, there is not any 4th letter. By way of example with the word “add”, there won't be any additional letters afterwards and therefore should be the word "add". The word “addict” is while in the BIP39 wordlist however , you might have used https://bookmarkick.com/story18866321/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting