. Alright, what you need to do is create another light and change the intensity (which is in the general tab I think) to a very small number, probably around 2-4%. Also set the shadows to Shadow Map (soft) and make the density around 75%.
. Next, put the new light in an array object. This should make a circle of lights which you should rotate and/or scale to fit around the objects your room. Now duplicate the array so there is a total of 4 array objects. Move each upwards slightly so you have a small cylinder, then scale the top 2 circles down so it makes a dome shape. This should be enough to give your scene some soft global shadows.
. It is possible that even with low intensity, your scene can get much brighter than you'd like. In this case, you just need to lower the intensity of your main light.
Hopefully some pictures will make this a little clearer
Effects on the scene (Will be somewhat different, since this is rendered in 3Ds Max)
There are probably much better techniques out there, but I'm a 3Ds Max man and don't know a ton about C4D. However, this technique is universal and should work well for you on top of being a simple way to get good, but fast renders.