You will need a UHS Speed Class 3 SD card to capture 4K video internally for the Panasonic GH4. Cards in stock here: http://www.bhphotovideo.com/bnh/controller/home?O=invoice&A=details&Q=&sku=1044453&is=REG

Panasonic SDUC32 GOLD SERIES: ULTRA HIGH SPEED CLASS 3 SD CARD / CLASS 10