For words that have only three letters, there is no 4th letter. As an example with the word “insert”, there isn't any much more letters Later on and so should be the word "increase". The word “addict” is from the BIP39 wordlist however , you would've used “addi” for “addict”. https://other-space.uncrn.co/blog/bip39-word-list-explained-your-necessary-resource-for-creating-secure-wallets-in-the-copyright-room/