Android uses a different version of UE4. The engine is built so that you can create one version of your game and port it as both a mobile version and a console/PC version but there are a lot of changes made between the two versions, including the type of rendering I believe. I'm not really an expert in UE4 but I have used it quite a bit so I'm somewhat familiar with the features.
Regarding new consoles, the truth is I don't know why everyone is so convinced there will actually be a PS5 and XB2 in 2 years. I can easily see this generation lasting as long as the previous one, especially now that the PS4 is a roaring success. But in the event that there are new consoles in 2 years, if they are targeting 4k standard, that really doesn't present much of a problem in scaling down because the Switch will still have a 720p resolution, so the first (and easiest) thing to cut will be resolution, and that will cut out a huge amount of the GPU load. From there it will be the standard scaling process, cut down texture res, LoD, view distance, effects, shadows, etc...
As for CPU issues, I really don't know if the PS5/XB2 (I really hope they don't call it the Xbox 2 by the way haha) will go for much better CPUs, but if they do that will begin to present a problem for Switch ports, yes.