You certainly don't need modern PC power even for 4k video, though you may need it for doing realtime decompression of some very high compression codecs (but that isn't "watching 4k video", it is "doing realtime h265 decompression for 4k video" or something along these lines).
Though codecs are one of the few cases where you see hardware being taken advantage of because that is basically their entire purpose.
Though codecs are one of the few cases where you see hardware being taken advantage of because that is basically their entire purpose.