you can downscale the 4K stream using the Image Signal Processor (ISP) the following way:
camRgb.setResolution(dai.ColorCameraProperties.SensorResolution.THE_4_K) # 3840 x 2160
camRgb.setIspScale(1, 3) # 1280 x 720
For more information about the
setIspScale method, see the documentation here.
Is this what you meant by "down converting" the 4K stream"?