Easycap002 4-channel video grabber

I have an easycap002 4-channel video grabber where I can connect 4 composite cameras.
It works with v4l2 on my raspberry, where I can select all 4 channels and show each individual stream.

In C++ I can get 1st channel with
cv::VideoCapture camera(0);

How can I capture all other channels (Composite1 … 3)?


Already found a external command to switch.
E.g. switch to channel 2:
pi@raspberrypi:~ $ v4l2-ctl -d /dev/video0 -i 2

I can send command by child process but smarter will be to find opencv function to switch.