The problem is that HEVC is encoded in H265 and there is no hardware support for that built into the ATV4. As a result the decoding has to be done in software and the ATV4 CPU is not up to the task. In fact at the moment inFuse struggles to handle 1080p in HEVC (although FireCore are working on that and are hopeful it will be resolved).
It is always possible that FireCore may find a way of optimising H265 decoding so that 4K can be handled but I would not bank on that happening.