Path tracing is like brute force. It gives highest quality and most realism, at the cost of speed. It is usually great for exteriors, because usually exterior feature mostly direct light, so calculating the cache would only add to the render time without much benefit.
UHD Cache speeds up the rendering by introducing some bias (simplification of the light interaction with the scene). It is mostly useful for interior scene, and scenes where GI is dominant compared to direct lighting.
4K Cache is similar to the UHD Cache in the way it works and how it should be used, however it has slightly different properties. Most of all, it should never flicker, even when calculating it from scratch for each frame. The pattern of the 4K Cache should also be more consistent when rendered on different computers. It should be a better choice for animations (be it calculating from scratch for each frame or appending to the existing calculation). It should also calculate faster.
We will definitely have more information, explanations, and use cases before the release.
