It's hard to be disappointed with any smartphone cameras, these days. Almost all of the modern smartphones take gorgeous video - more than any conventional consumer could ask for. Even those who consider themselves high-caliber photographers are perfectly fine with phones that came out more than three years ago. That said, whenever a technology can be improved, there will always be people who want to be on the bleeding edge of it. For those people, SuperHDView (via Reddit) has put out this video comparing the LG G4 and the Lumia 950 XL's cameras in 4K.
While both of these cameras look fantastic when compared with one another, there are definitely some subtle differences between them, which the commenters on the video are all very astutely pointing out. People have claimed that the G4 has better dynamic range, focus, and a wider angle. The wide consensus, however, is that the Lumia 950 XL has more pronounced colors. Most people also agree that, while the G4 takes slightly better video, the Lumia 950 XL is probably better for photographers.
Either way, you're going to be happy with whatever camera you have. Both of them are exceptional for whatever purpose you might need them for. The 4K video looks amazing either way, and I find it unlikely that anybody would be disappointed with the video that they get from either phone.