For words that only have 3 letters, there is not any 4th letter. Such as with the word “insert”, there are no far more letters afterwards and thus needs to be the word "add". The word “addict” is during the BIP39 wordlist however you would have made use of “addi” https://franciscougju442.bravesites.com/entries/general/Opening-the-Power-of-BIP39-An-Extensive-Consider-Word-Lists-and-Secure-Budgets