The trick is using the right software with the right configuration. I have tried it on a $500 AMD A6-3400M laptop with a 5400rpm drive and 4GB RAM and no discrete GPU. 4K X264 @ 200 Mb/s files work just fine. Remember, 90 Mbit is only 11 MB/s, which is pretty easy for even the slowest of hard drives, so no RAID or SSD required.
Of course, if you try playing with VLC you will only see a slideshow.
Here's what works, and should on average computers running Windows 7:
Frontend: Media Player Classic
Renderer: Overlay Mixer (the default is EVR which will work on reasonably powerful computer, but for laptops and such this needs to be changed. If you have a powerful GPU, MadVR will work too for excellent quality)
Decoder: ffdshow DXVA (If you have a quad core CPU, go for ffdshow software minus DXVA)
Intel IGPs are not going to work. If you use OS X, Quicktime won't work at all and VLC will require a powerful system like the ones you mention. The best bet would be MplayerX but I haven't found it to be very stable and would still require a quad core CPU. On Linux, use any ffdshow frontend and disable VDPAU.
4K display is a different matter, but they should go mainstream by 2013 (starting late 2012). The good news is every new GPU or APU released in 2012 supports 4K outputs.