I've never made postcards, but I've made reports for printing 2-sided business cards and convention badges, and the concept is similar.
The general idea is to create a report where the page size is equal to the size of a single sheet of your postcard stock (in this case its a normal page size of 8.5 x 11 with portrait layout). Then in your section expert for your details section you will want to check the box on the Common tab to enable "Format with Multiple Columns". This should add a Layout tab in Section Expert where you can set the size of each postcard on the sheet. This should allow your report to know there will be 4 cards per page.
Right click on your details section and create a second details section after the first. Then enable the New Page After property of the first details section. This will force the first details section to be page 1, repeated 4 times, then the second details section becomes page 2, repeated 4 times.
The front of the postcards will be designed in first (page 1) details section, while the back of the cards will be designed in the second (page 2) details section.
When you print your report, make sure you have 2-sided printing or full duplex turned on so that it knows to print on both sides of the paper stock being used.
This should be enough to get you started!