For words that have only 3 letters, there is absolutely no 4th letter. One example is with the word “insert”, there aren't any more letters afterwards and therefore should be the word "increase". The word “addict” is from the BIP39 wordlist however you might have made use of “addi” for https://socialtechnet.com/story4282266/a-secret-weapon-for-bip39-words-to-numbers