Hi DouglasSpyrison,
Welcome to the Microsoft Community.Thanks for supporting Microsoft products.
According to your needs, you want a postcard template that is editable on both sides, letter-size, 4 postcards per page, and can be edited on both sides.
If you have Publisher, it is recommended to work in Microsoft Publisher (more suitable for professional typesetting). If you have not searched for a suitable officially published template in the application, in Microsoft Publisher you can create a customized document. First, open Publisher and select a blank document, set the page size to letter size (8.5 x 11 inches). Next, you can help align the 4 postcards by inserting grids and guides and adjusting the size of each postcard, for example setting it to 3.5 x 5 inches.
On the first page, you can begin designing the front of the postcard by inserting images, text, or other design elements. When the design is complete, insert a second page for designing the back of the postcard, where you can add address boxes, stamp locations, and more.
Once you've completed the design, you can view the results in Print Preview, make sure both sides of the design are aligned, and print the second side using the duplex printing feature or manually flip it over.
Perhaps also try visiting some of the third party template platforms to see if there is a more suitable template for you, which you can download and check that the page size matches your requirements.
I hope this is useful and if you have any questions please let me know.
Best regards,
Ian - MSFT | Microsoft Community Support Specialist