Man, I think that PC has had enough torture. I'd drop those cloud samples down to 196 and atmosphere samples down to 64. And knock that resolution back down. 4000x3000 is just too legendary of a size for the current version of TG2 and hardware.
Additionally, do you have ray traced shadows on? I turned that on once for a scene that could have used it (due to mountains blocking the sun), and the render times jumped by a factor of 4 or more.
Anyway, my longest render was close to a week - and the image sucked when it was completed, and it was at a paltry 800x600 resolution.