Here is an example in order to capture 4K content under Windows:
>> ffmpeg -f dshow -rtbufsize 1500M -video_size 3840×2160 -i video=”1714-INOGENI 4K2USB3″:audio=”Digital Audio Interface (1714-INOGENI 4K2USB3)” –q 4 output.avi

Replace the device names by the ones listed when you issue the following command:
>> ffmpeg -list_devices true -f dshow -i dummy

in 4K2USB3ffmpeg Tags: 4K2USB3