A backflow preventer is a safety device often required to protect city water mains from contamination.
They can be anywhere from one inch to 10 inches diameter. Kbeck works on all sizes. What you're seeing here is a 4-inch backflow preventer that was assembled in our shop for speed and quality of workmanship and then taken to the site. There was a reason for this. This was an eight-hour job and the water was off for that time. It couldn't be off longer since this was in a mobile home park and the line served 300 units.
The hallmarks of quality here are that it was cleanly installed, upright and not leaning at all, important because the device is top-heavy. Installed after the photo was taken was a heavy steel unistrut spanning the box and ensuring the installation's stability.