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I was trying to find the technical reasons for why H.264 is limited for 4K. Obviously that in terms of bit-rate it becomes less efficient but the encoding itself is limited and for some reason, I couldn't find any publicly available references.
Someone?
One of the bits of information associated with a H264 stream is its level. The level informs the decoder the computational resources needed for a successful decode. Turns out that the highest level 5.2 supports upto 9437184 luma samples per frame, which is the number of luma samples needed for a frame of size 4096x2304. So, there may be encoders that can (be forced to) encode higher resolutions but there's no valid level which can be set for the stream, and probably no decoder that can be guaranteed to produce reliable playback.
H.264/AVC is not limited for 4K in terms of max resolution as it supports DCI 4K (4092x2160) @ 60 fps at Level 5.2. For example Sony already uses XAVC which is exactly 4K AVC.
For a while the max level was 5.1 and 5.2 was introduced later. If needed new levels can be introduced as long there's the hardware to support it.
For eg. the Nvidia NVEC supports H.264 up to 4096x4096: https://developer.nvidia.com/sites/default/files/akamai/cuda/files/CUDADownloads/NVENC_AppNote.pdf
The actual problem with 4K are the bandwidth requirements for streaming, not the hardware support. To do 4K streaming at 12-15 Mbps the encoder needs a 40% efficiency increase.
The problem with H.264 is that it's not efficient enough for this purpose. This is what the new H.265/HEVC (High Efficiency Video Coding) tries to achieve.
