Are there battery grips available to house 4 batteries for Canon? Is there any other way I can keep my camera powered for long periods of time?
You can only buy two battery camera groups for the Canon 6D. Have a look at the Canon BG-E13. This let's you use two batteries which effectively doubles the battery life of your camera.
You can also buy a mains adapter for the 6D which substitutes the battery.
If you want to try a DIY option you could combine the Battery adapter part of the 6D mains coupler (remove the AC-DC converter) and then attach it to a bigger battery. Say a 7.4v Lead Acid. However you might want to build or find some circuit protection to avoid it damaging your camera.
Have a look at this article here where someone has done something similar: http://starcircleacademy.com/2011/01/all-night-power/
Also take a look at this thread which tries and finds a similar solution for a Canon 5D: http://photography-on-the.net/forum/showthread.php?t=1383145