Will I consume 2G/3G/4G traffic when I use the mobile app to view real-time images, download videos or play back videos in the Yi Dash Cam-Nightscape?

will not. The Yi Dash Cam-Nightscape itself has a built-in WiFi module that can create a WiFi hotspot, which is equivalent to a LAN WiFi. The mobile phone and recorder use a point-to-point private network connection and do not consume 2G/3G/4G traffic without going through the 2G/3G/4G network.