5k rendering won't start

[capmotion]

I'm trying to render a 500k polygon scene at 5700 px wide, and it won't start. But when I lower it to 5400 px it starts. I have 2x GTX 970 (4 Gb).
Did I hit the memory ceiling of my setup, or why won't the render start at the higher rez?

Thanks.

[itsallgoode9]

when you're running the lower resolution that starts, how much vram does it say you're using in the IPR window? That'll let you know pretty quickly how close to your limit you are. 500,000 polys doesn't seem like it should be using that much ram. are there lots of textures? what about render passes, do you have alot of those turned on?