I am strongly considering using UE4 for my next project. I’ve used UDK in the past, but there are some things I am curious about.
First of all, I have some questions about the license.
Let’s say I buy a subscription for a while, and then cancel my subscription. Will I still be able to use the engine? Would I still be able to release a game I made with the engine, and sell it?
Second, I have some general questions about cross-platform options.
What kind of work is actually involved with porting a game onto multiple platforms? I’ve worked with Unity, and in there I just go into the menu in the editor and select what platform I want, and the game will automatically build for that platform. I can easily change something in my game and then produce builds for every platform I am working with. But how well does work within UE4? And how does UE4 handle downscaling of visuals for platforms like mobile?
Welcome to the forum! 
I have some good news for you:
1 - Yes you will still have the engine version you downloaded when you subscribed, and if you cancel, you will not receive any updates, but you are free to use the engine version you downloaded for as long as you like, and also release your game made with that version as well!. 
2 - The workflow you described above, is exactly the same as the way you would package out to different devices. You simply click build from teh file menu, select platforms you would like to support and start building, it is as simple as that!!
As for the downscaling of visuals, you will need to mark some of the materials you create as mobile ready (I think it does that by default though), but if it isn’t default, it is just switching between modes on the material, or mesh etc. As well as the editor in general can be scaled so that you see on screen how it will look in your released game.
Overall, I don’t see anything that might hold you back, although the current compatibility for Android devices is still being worked on, but aside from that is should work great for what you are looking for!
Well that’s good to know; thank you!