I have been using Atomos Ninja to record uncompressed video from my A7S for a while now and can’t speak more highly of the product. If you can’t, however, yet justify the purchase of an Atomos Shogun (7-inch external recording device and monitor) to record 4K footage from the Sony A7S, Michaels cameras in Melbourne, Australia have one available from their hire department. They also have an A7S for hire if you want the whole package. If you intend to record extended 4K movie clips using an A7RII, I would also recommend using a Shogun. It will prevent your A7RII sensor getting hot and also give you the advantage of a big screen for accurate focus pulls.
The Shogun provides a stunning 7.1” 1920 x 1200 (320ppi) screen with a good viewing angle making it perfect as a director’s monitor. With class leading features like waveform, focal assist, zebra, calibration and 2:1 zoom it’s the perfect tool for perfect shot setup.