Last Four is a concept at this point. We are working on a new feature to support multiple account numbers, seamlessly. For example if you have a credit card (ending 2168) and add it to your phone it gets a new number (ending 4149). Then when you add it to you watch it gets another (ending 4156). This makes life hard for Accouting when submit expense receipts and the have three different Last Four's for your card.
This Solution will provide details. (6783)
Last Four is a concept at this point. The idea is simply to key the last four and find the right Account. To clarify, this applies to purchases. For sales the Credit Card widget already supports keying the Last Four to find the matching card.