For words that have only 3 letters, there is absolutely no 4th letter. One example is with the word “insert”, there won't be any far more letters afterwards and thus need to be the word "incorporate". The word “addict” is in the BIP39 wordlist however , you might have made https://crossbookmark.com/story19363458/detailed-notes-on-copyright-wallet-seed-phrase-out-of-bip39