A mobile DVR (Digital Video Recorder) with 4G, HDD, GPS, and SD card support is a versatile device typically used for vehicle surveillance. Here’s a breakdown of the features:

1. 4G Connectivity: Allows real-time monitoring and video streaming over cellular networks. This feature ensures that you can access live footage remotely, making it ideal for fleet management, transportation services, and security monitoring.
2. HDD Support: The device supports a hard disk drive (HDD) for extended storage capacity, enabling long-term recording. It is useful when storing high-definition video footage for an extended period.
3. GPS Integration: With GPS, the mobile DVR can track the vehicle’s location in real-time, embedding geographical data in the video recordings. This is especially useful for tracking vehicle routes and improving fleet management.
4. SD Card Support: In addition to the HDD, the DVR supports SD cards, providing additional storage options. This can be used as a backup storage or when quicker access to recorded data is needed.

Such a device is perfect for use in buses, trucks, taxis, and other vehicles for security and monitoring purposes.