Not exactly what you're asking, but I use a machine with an Intel i7 6650U/Iris 540 to upscale 720p or 1080p to 2736 x 1824 (not quite 4k, but half way there) daily. It works perfectly with RO Standard without breaking a sweat, and is close to working well with ROHQ on normal settings (occasional frame drops).
It's hard to assess what "100% GPU utilization" means with the intel iGPU's because they share a package (and heat) with the processor, so if the processor is very active the GPU may wind up throttled (power or thermal) where it otherwise might not (there's a similar tradeoff with shared memory, etc.). So the GPU may be able to "do more" when the processor itself is doing less if you follow me, depending on your cooling situation and form factor.
I can't test at all as I don't have a true 4k screen, but I would be willing to bet that the Iris would be fine for RO Standard at 4K, but not even close for ROHQ. Even more so if the device you're contemplating is a conventional desktop pc or a larger laptop with robust cooling. My device is a Surface Pro tablet, so the heat dissipation is necessarily limited by the small form factor, which limits performance.