I have an idea that is worth trying(i think) If you are rendering a large scene do one part in one file and another on the another document and stitch it together in Photoshop or whatever you use. I don't think you can get past it otherwise as 4000Px is the max recommended and I tried putting 5000px in and it will automatically go back to 4000px. Or you could enlarge it in Photoshop choose Bicubic Smoother(best for Enlargement) and whack up the resolution to 3000. 

Hope that helps a bit.

 

Karl