For words that only have three letters, there isn't any 4th letter. For example with the word “insert”, there aren't any more letters afterwards and so has to be the word "include". The word “addict” is from the BIP39 wordlist but you might have made use of “addi” for “addict”. https://sociallawy.com/story9246094/not-known-facts-about-bip39-english-words