convert 1080p to 4K in CC give me error.

Hi

I am using after effects CC

I import an AVCHD 1920x1080p 50p 28 Mbps clip , then I use the effects details preserving upsacle in order to upscale it to 4K.

The problem occurs when I try to export it to h264 mp4 at 4k maintaining the 16/9 ratio 4096x2304 with a bitrate of 50 Mbps ,  an error message says "Output size will be resized from 4096x2304(1,00 PAR)  to 1636x2304 (2,50 PAR) to meet format constraints.".

Why?

How can I export my original clip in a 4K 16/9 ratio with the effect detail preserving upscale ?

Thanks