Toshiba Exceria 64GB SDHC SDXC Memory Card UHS-I U3 4K FHD Up to 90MB/s
Looks like a good price at the moment for U3 high speed SD card that's suitable for 4K video recording.
Review is quite positive as well.
Blurb from the website:
XCERIA™ - SHOOT 4K VIDEO WITH CONFICENCE
Enter the world of 4K and Full HD video and highspeed photography with the Toshiba EXCERIA™. Great for aspiring photographers, the SD card series provides the speed and capacity needed not only for image and video capture, but also for efficiently transferring and editing the results. That‘s thanks to the read speeds of up to 90 MB/s and up to 128 GB capacity.
Always in Your Pocket
Put the latest Toshiba Exceria SD Card in your digital camera or your mobile device and expand your storage at an affordable price. Now you can take hundreds of pictures with your smartphone or store songs and videos.
Capture Your Precious Moments
Great for enthusiastic photographers, the series provides the speed and capacity you need to catch the perfect moment in Full HD.
Secure your file
With the Toshiba write-protect feature, you can prevent accidental overwrites. Also, no worries at the airport security check, the cards are x-ray proof to protect your precious memories.
5 Year Warranty
Engineered to Toshiba’s famously high quality standards, the Toshiba SD Memory Cards are backed by a solid five-year limited standard warranty.
Back in my day you'd flip a disk over to double its capacity