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 the trees in PFX keep the polycount lower. If you don't need 4k don't use it 1080 is normally enough.
If you need 4k consider adding a bunch of cheap render nodes. And definitely add more ram MR will love you for it.
David
From a readers' Q and A column in TV GUIDE: "If we get involved in a nuclear war, would the electromagnetic pulses from exploding bombs damage my videotapes?"