For words that have only 3 letters, there is absolutely no 4th letter. As an example with the word “include”, there aren't any extra letters afterwards and therefore must be the word "add". The word “addict” is from the BIP39 wordlist however you would've employed “addi” for “addict”. This text https://businessbookmark.com/story4269570/bip39-phrase-generator-can-be-fun-for-anyone