Sure, but they wouldn't implement H.265 software encoding for 4k, likely the A9 would struggle or at the very least it would wreck your battery life. There's speculation as to whether or not the A8 (and subsequently the A9) has hardware encoding/decoding for H.265 but that was centered around FaceTime using it. FaceTime is only 720p though so software decoding would certainly be feasible and is most likely.
I suspect the real reason they are using H.264 for 4k is compatibility, I don't think OS X even has native support for the H.265 codec, Windows 10 does have native H.265 codec but not Windows 8. Processing power is still an issue for with software decoding and hardware acceleration is very limited as of know. For Nvidia only 960 and better cards have full hardware support for H.265, AMD discrete cards don't have H.265 support but I believe some APUs do, for Intel I believe only Skylake has full support for H.265 (Broadwell and Haswell are only partial).