As the new Panasonic GH4 (and other 4K cameras) start shipping, your typical SD card may work for 1080p resolutions, but If you've been waiting to shoot 4K Video as soon as your camera hits your doorstep you'll need media that packs in whopping speeds.
The new Sandisk Extreme Pro SDHC and SDXC media are optimized for UHS-II Devices, holds a speed class rating of UHS Class 3. Rated at a Max. Read Speed: 280 MB/s and Max. Write Speed: 250 MB/s, these are going to be cards required to take advantage of that 4K data rate. The speed of these cards will also greatly benefit you when it's time to dump or backup the data from the card, so you can get back to shooting.
If you think you can get away with a cheaper set of SD cards keep me in the loop, but for the Panasonic GH4 (and most likely other 4K camera) 'these are the cards being recommended' by many early reviewers. As with anything related to 4K media and storage, they aren't cheap starting at $75 dollars for the 16GB SDHC (approx 10 minutes of 4K video) to $245 for the 64GB SDXC Card.
Sandisk Extreme Pro USH-II [Class Speed 3] SDHC / SDXC Media for 4K Video