Apple released a new beta version of the 16.4 firmware designed for the Studio Display alongside the second beta version of macOS Ventura 13.3 this week. The beta arrives fourteen days after the first version of the 16.4 firmware was made available for testing on February 17.
All Studio Display models can get firmware updates wirelessly over the air. However, the macOS Ventura 13.3 beta is currently required for this firmware update, so it is only available for Macs that meet those requirements. Studio Display owners running macOS Ventura 13.3 can install the firmware by going to System Settings > Software Update.
Apple has not provided any additional information regarding the contents of the firmware update.