For words that only have 3 letters, there is no 4th letter. As an example with the word “add”, there are no extra letters Later on and so have to be the word "add". The word “addict” is during the BIP39 wordlist but you would've applied “addi” for “addict”. A https://thebookmarknight.com/story19016446/bip39-phrase-generator-can-be-fun-for-anyone