I just received the new Atomos Shogun 4K Recorder / Monitor and obviously I plan to pair it up with the Sony A7s to utilize it's 4K Video feature (via HDMI only), and also test out 10Bit 4K via the GH4.
For something this heavy, it's probably not a great idea to mount it directly to the Hot Shoe of the camera, so you'll need to look into using some type of accessory mount or Camera Cage. Here's a look at two Varavon Armor Pro Cages designed for the Sony A7s and Panasonic GH4.
Instead of a fairly large generic shell, each Armor Cage is custom designed only to fit a specific camera. This allows the cage to be remain very slim and allowing full access to the camera's dials, inputs, and doors. Each Varavon Armor Cage comes with a leather Hand Strap, while some packages offer a Top Handle and optional rotating ENG Hand Grip.
To maximize the run time on these cameras (I really need one for the Sony A7s), Varavon also offers external battery power solutions for many types of cameras.
Varavon Sony A7s and GH4 External Battery Package
For more information about these cages and optional accessories, check out the Varavon website (here).
Varavon Armor Cages for Sony A7s Pro and GH4 Pro