How would I add the last 4 of credit card to the payment transaction failed reminder email?
You can get that information from "sales_flat_order_payment" and could be other tables. But in order to send on reminder mail you need to write piece of code. Also pl take note Magento do not store CC info if you used authorized.net or paypal. Yes of course store exiting CC method info.
Hope you have idea about PCI..