Reflections are basically done in two parts in UE4. SSR is the main dynamic reflections, however the engine can fall back onto capture actors to fill in the spots SSR misses.
Capture actors capture a cubemap and use that to provide local reflections relative to the actor. Grab a sphere scene capture actor and just drop it somewhere, along with a skylight set to movable, and you should see the capture actor providing static reflections.
The only solutions you have available are SSR, precomputed cubemaps in materials to manually fake the reflections (Alan used this to give some reflections on the translucent water surface in the water planes pack), or scene capture actors. SSR uses the pixels on-screen to produce reflections, hence why it’s called a screen-space reflection. Because of this, anything that’s off-screen isn’t reflected, SSR can’t even comprehend the concept of off-screen due to the fact that it’s a screen-space effect. This is why when you look down, reflections break. SSR doesn’t know what’s above the floor and hence cannot reflect what you’d expect to be reflected. Scene capture actors work by capturing a cubemap, basically a big cube with textures pasted onto the inner walls, so whichever direction you look at within the cubemap is the same as what it’d be in the actual scene. The reflection environment can use capture actors to fill in world-space detail (reflect things off-screen that SSR cannot reflect), but it’s lower quality and static, so the reflections do not update in real-time (unless you use the dynamic capture actors which destroy performance).
Basically, what you do is you use a box capture actor to encompass the entire scene, then where there’s smaller detail, say in corners or near important parts of the scene like a desk, place smaller sphere capture actors. Smaller capture actors take priority over larger actors, hence you can increase the quality and accuracy of reflections where needed. Remember that the reflections are taken from the point of view from the actor itself, so position all your actors in such a way that it correctly reflects the scene. Then, you can use SSR to provide more accurate screen-space reflections.
If you want, you can open up the reflections demo scene available from the launcher and see what I’m talking about within the engine already done.