As more people receive their GH4 cameras, more tests are being performed across every feature imaginable - including a slew of various media cards. I've already shared some great options previously, but here's another set of SDXC cards shared by Jay [Thanks Jay].
These Transcend SDXC Cards have a Read speed of 95MB/s and Write Speeds of 60MB/s. Perhaps not the fastest card available, but definitely one that falls easily in the 'Best Bang for the Buck' category. At just $48 bucks normal price for a 64GB SDXC Card you're looking at around .75 cents/per GB. You can find these new Transcend SDXC Cards following the link (click here).
Transcend 64GB / 128GB UHS-1 SDXC Memory Card
BTW, another change from the GH3 camera is the HDMI out port. They are no longer using the Mini-HDMI connection, instead they are using the microHDMI D-Type. So I ended up having to buy some new cables for my ATEM switch. In case you're still waiting for your GH4 to arrive, you may want to get your microHDMI cables in too. I picked up a set that had over 1,000 reviews, and were affordable. Sounds like they will work just fine: http://www.amazon.com/Cable-Matters-Speed-Micro-Ethernet/dp/B004C3AW40/.
D-Type microHDMI Cable 3ft-25ft Length