Hey, I just thought I’d chime in, as pushing the quality of projects for Archviz is something I’ve been playing around with.
You have to remember, if you have a 2K or 4K image, that image is being spread across 360 FOV, when your camera view is probably around 90 or less, the result is not going to look crisp.
Here’s the steps I take to get really crisp HDR images in the background…
I start with a 16K HDRI file. I will generally chop this into 2 8K Squares, so I have 2 8192x8192 images. Save these images as a .EXR, so you still maintain the high dynamic range, but the engine doesn’t import them as a cubemap.
Follow this advice here to max out texture resolution to 8K: How to set in game texture resolution to 8K? - UE4 AnswerHub
Make 2 standard unlit materials, with one half of the sky as a texture for each material
I know you said you were a little unsure about UV management but you’ll need to make a custom sphere that has 2 separate material ID’s one for each half, UV’d properly so as to accept a square image for one half of the sphere. If that’s a little cryptic, I’ve put it here: https://drive.google.com/open?id=1Ym6MMHuZx2hzvexn6ySsyi9Nc9SyVsRn
Keep in mind this will eat a huge chunk of texture memory, as you are essentially loading a 16K texture at 32 Bit, but it really makes those still archviz renders or videos pop IMO