Comment # 12 on bug 93405 from Pierre Moreau

I highly doubt your card could output 4K (maybe on perflvl 0f, but for
reclocking on that card you need kernel 4.4), and even if it did, I don't think
it supports HDMI 1.4 or DisplayPort 1.3(?) which are needed to support 4K at,
at least, 30Hz.