For words that have only three letters, there is absolutely no 4th letter. By way of example with the word “include”, there are no more letters afterwards and thus have to be the word "insert". The word “addict” is while in the BIP39 wordlist however , you would've used “addi” https://other-space.uncrn.co/blog/bip39-word-list-explained-your-necessary-resource-for-creating-secure-wallets-in-the-copyright-room/