For words that have only three letters, there is no 4th letter. For example with the word “add”, there aren't any a lot more letters afterwards and therefore should be the word "add". The word “addict” is in the BIP39 wordlist however , you would have utilized “addi” for “addict”. https://bookmarketmaven.com/story19582727/little-known-facts-about-bip39-seed-phrase-list