Without knowing which version of Photostory you have - you should always state the name and version (in full) of the software you are using - it is difficult to give an accurate answer - not all versions support 4k.
However if the version you have supports 4k export then you select the MPEG 4 export option and choose one of the UHDTV presets.
There doesn't appear to be any way of saving a 4K file to my computer. However there is an option to upload to YouTube in 4K but I have never been able to make that work. It always says there was a problem uploading.