For words that have only 3 letters, there is not any 4th letter. As an example with the word “add”, there won't be any much more letters Later on and so have to be the word "increase". The word “addict” is in the BIP39 wordlist however, you might have employed https://other-space.uncrn.co/blog/bip39-word-list-explained-your-necessary-resource-for-creating-secure-wallets-in-the-copyright-room/