Hey! What I'd try to do is use the UE4 Media Player framework (https://docs.unrealengine.com/latest/INT/Engine/MediaFramework/), create a sphere with a custom shader, put your camera inside the sphere and basically raytrace the video onto the sphere. Most 360 videos out there are lat-long (or equirectangular, same thing) encoded, which maps really easily onto a sphere (http://mathworld.wolfram.com/SphericalCoordinates.html).
The main issue here is that the MediaFramework is not officially supported by UE yet and is "still in the experimental stages". We're also working with the UE developers to publish at some point a VR template using the Media Player framework.
