With near identical specs as the GoPro HERO4, the Yi 4K is basically a half-priced version of the action cam king.
There have been plenty of cameras that have come in and tried to take over the action camera market, but for years GoPro has held on strong to their place at the top. But where others have failed, the Yi 4K action cam might just win, because instead of going for reinvention or specialization, it's going for straight up imitation. Essentially, it's giving the customer what it wants—a GoPro.
The Yi 4K has almost identical specs as the GoPro HERO4, including its processor, sensor, and lens. It also records 4K at up to 30fps (like the HERO4), has an auto low-light feature (like the HERO4), has Wi-Fi (like the HERO4). At first glance, this thing looks like a HERO4—and apparently sounds like a HERO4, according to Brent Rose of Wired, who reviewed the camera. (The whole thing is kind of—Dostoyevskian, right? Like, one day the GoPro Hero4 is going to turn around and see its exact double taking over its life.)
To be fair, though, Yi 4K has a few beefed up features not found in the HERO4, like a built-in 2.19-inch retina touchscreen display, 120 minutes of battery life (though Rose's test reports 2hrs, 48mins), a 3-axis accelerometer and 3-axis gyroscope, EIS, dual microphones, slow motion mode, lens distortion correction, and a tripod mount on the bottom.
Here are a bunch of videos to give you an idea of how the Yi 4K performs:
So yes, this thing is more or less a complete knockoff of the GoPro HERO4, but a knockoff that 1.) costs half as much as the original—which most knockoffs do, and 2.) actually has a bunch of impressive features that the original doesn't have (though, it has fewer shooting modes, no Superview, and no 24fps capture). Of course, the worry with things like this is that the construction or image quality is subpar, but according to Rose, they appear comparable to those of the GoPro HERO4.
You can purchase the Yi 4K Action Cam off of Amazon for $250 (the bundle also includes a sweet selfie stick and a Bluetooth remote). And like GoPro, they offer a ton of accessories, like waterproof housings, handlebar grips, and a ton of different mounts for your chest, head, wrist, etc.