Do not download a "4K Half-SBS" file. It is a waste of space. Half-SBS halves the horizontal resolution. If you want true 4K 3D, look for or "Full SBS" (3840x1080) . For most VR headsets, a high-bitrate 1080p Half-SBS is perfectly fine.
Finding legitimate sources for 3D content is harder than finding 4K content, but not impossible. Here is where you can source your movies. Online 3d Sbs Movies-
However, there are two variations you need to know: Do not download a "4K Half-SBS" file