As far as I know, only cameras can be used to test using darknet_ros. So, how can I test with mp4(video) file?
The way darknet_ros comes out of the box, you are correct. It subscribes to an sensor_msgs/Image topic and uses that as input. If you're trying to use this with an mp4 file you need to get that file publishing out as a video over ros. Right now the best, and really only, way to do this is via an opencv package. You can use the provided launch file on that page and change the input file directory. After that all you need to do is remap the output topic so it connects to darknet_ros