If you see you ram doing this you need more. Excuse the crude paint over :)

PS

Check the task manager if it is as i suspect ram, lower the polycount of the trees and grass, IE use less geo for the grass you can thin it out quite a bit and keep the same look. When making...

At 4k my guess, you have run out of ram. Mental ray will take days to finish a render if it runs out of ram. That scene with everything in it at 4k will take about 20gb to render efficiently there...