Panasonic introduced two 4K camcorders at the beginning of 2015--the WX970 and the VX870. The difference between the two is simple: the WX970 has a built-in secondary camera while the VW870 does not. Opting to forgo the extra camera will save consumers about $100.
The Panasonic VX870 still offers 4K video at 30 fps, recorded on a 1/2.3" sensor. The Sony AX33, introduced at the same time, uses a larger 1" sensor, which typically translates into better low light performance, even if the resolution is the same. The VX870 still sports a 20x zoom with optical image stabilization. Like Panasonic's other 2015 cameras, high dynamic range video is also possible, though only in HD and not in full 4K resolution.
While the VX870 doesn't have the built-in secondary camera, you can still achieve the picture-in-picture effecting using your smartphone's camera, thanks to the Panasonic app and wi-fi. The downside is it's harder to hold a phone and the camcorder at the same time, though the secondary footage is less restricted since it doesn't have to stay right next to the camcorder.
Overall, the VX870 looks like an excellent option, and one of the most affordable 4K camcorders on the market currently.