I have 4 cameras in different positions but all of them point to the center of the scene. What I want is to combine all the renders, like film taken from different cameras, combined in one, image from different angles? Is it possible to render all of them at once and combine the render result into one image?
You can do this with multiple scenes.
Once you have completed your scene create a new one, selecting Link Objects. This makes the new scene use the same objects as the original, when you move an object it will move in each scene.
You can either delete the camera in the second scene or just change the active camera for each scene.
If you add a new object after you duplicate the scene, select the new object, press CtrlL and choose the scene to link it to.
Then in the compositor you add multiple RenderLayer input nodes and choose a different scene in each one and composite as you choose. When you render each scene will be rendered automatically.
This only works automatically when you have 4 cameras/renders with the same resolution. i cant make it work for animation with 2 cameras side by side. Like cam1, cam2 has 960x1080 and the final render should have 1920x1080. The resolution gets overwritten by the composite scene (cam1, cam2 will have 1920x1080) when you press F12 for render. However if you then click in the node editor the small render button for each render layer it works.
The only general way i found was to render them separately and then combine them in the movie editor.