First things first. The G3258 will not display 4K video via the Integrated Intel HD graphics. Even with the DP connection, max resolution is 2560x1600p.
Worse yet, the chip has purposely disabled hardware H.264 decoding assist (Intel Clear Video HD).
It's a decent enough CPU for a HTPC, but you're really going to want to add a discreet GPU to the HTPC if you intend on 4K. I'd stick a low-power solution in there, something like a GTX 750 Ti so you can keep the noise, power consumption, and heat down to a minimum - assuming the HTPC is going to be on 24/7.
If you step up to a i3-4160, you can do what you want without the discreet GPU, but the chip costs 2x the price of the G3258. Still cheaper than adding a GPU.
GregEdited by hammong - 11/11/15 at 6:17am