Is it possible to combine renders from 4 cameras into a single 2x2 split-screen static image? (with single "Render Image" action)  

Using the Stereoscopy feature we can render from several cameras with single F12-hit. Using Scale/Translate/Mix nodes we can also arrange images over/next to each other.

But I don't know how to point to the render output of particular camera in the Compositor. Is that even possible?