Well for most things just about any uSD card should be adequate, but 4K video is going to eat a ton of data really quick. I believe we're talking 3X-4X the rate for 1080P video and that's about 10GB/hour so we could be talking 40GB/hour -- or about 11MB/sec.
Of course, a storage devices write speed isn't just about raw write speed of a large file/files but also takes into account writing to many smaller files. When I copied my music to the 128GB uSD card last Tuesday I did so with the card installed in the Note4 and I figure the sustained data rate had to be about 13.65MB/sec so if thats representative of the cards write speed then the worst case writing 4K video should be OK.
Brian