For words that have only three letters, there is no 4th letter. For example with the word “add”, there isn't any far more letters afterwards and so should be the word "add". The word “addict” is from the BIP39 wordlist but you would've employed “addi” for “addict”. A seed phrase, https://getidealist.com/story20564471/5-simple-techniques-for-bip39-word-list-pdf