For words that only have 3 letters, there is no 4th letter. By way of example with the word “insert”, there isn't any much more letters Later on and so needs to be the word "include". The word “addict” is within the BIP39 wordlist however , you would have utilised https://wisesocialsmedia.com/story4442904/the-greatest-guide-to-seed-phrase-bip39