Heightmaps are NOT art work.
You cannot filter/alter them the same as you would an image.
Also you shouldnt alter the scale settings unless you know what you are doing. With a value of 61.decimal places i seriously question your knowledge of it and related setting of this value.
Third. The maximum size for a heightmap or any engine file is usually 8k.
You exceeded that by its double. And if you were able im guessing the heightmap is probably not in png16 format (but the engine could have changed to allow bigger files - however thats why your computer melts).
Fourth.
2x2 vs 1x1 = number of total components = draw call count per tile = performance.
2x2 is less calls. Usually better for flat terrains.
1x1 is more. Could allow for more culling in steep canyons.