For words that have only three letters, there isn't any 4th letter. For example with the word “incorporate”, there won't be any much more letters Later on and so should be the word "insert". The word “addict” is within the BIP39 wordlist but you might have made use of “addi” https://tbookmark.com/story19033405/not-known-factual-statements-about-bip39-explained