Below is a link to the current firmware update file for the DSR4 receiver. This is a zip file, after downloading extract it into a temporary folder. Use the Wireless Designer™ program to update firmware, instructions are found here.
Note: This is a link to the Wireless Designer firmware update file, which bundles the latest firmware for multiple products. Wireless Designer will automatically choose the right firmware images from the file. To update firmware in the DSR4 receiver, Wireless Designer version 2.0.43 (Windows) or 2.0.40 (macOS) or higher is required.