For words that only have 3 letters, there is not any 4th letter. By way of example with the word “insert”, there are no additional letters Later on and therefore must be the word "add". The word “addict” is in the BIP39 wordlist but you would've applied “addi” for “addict”. https://bookmarkswing.com/story20407249/not-known-facts-about-bip39-english-words