For words that only have three letters, there isn't any 4th letter. As an example with the word “increase”, there won't be any additional letters afterwards and thus need to be the word "add". The word “addict” is from the BIP39 wordlist however you would have applied “addi” for “addict”. https://bookmarksea.com/story19579025/not-known-factual-statements-about-bip39-explained