1- Use shadowplay to record and avoid all problems (if you have a nvidia card), Otherwise try what said.
2- 4096 will take too much memory, the problem is your ceiling is too big for a single lightmap. Break it in several parts, maybe 3 or 4 and try with a 1024 resolution on each parts. But we also need to know what are your lightmass settings! Can you share 'em?
3- You have to fake caustics with a light function. This is a way of doing it : https://forums.unrealengine.com/showthread.php?67673-Water-Caustics Because unreal cannot calculate caustics in real-time!