The main "trick" is to size up the terrain first to 4K in the terrain editor and the immediately SAVE the scene so Vue can create the necessary file buffer space and avoid a crash.

Then load (which I will explain further down) a 4K x 4K terrain file that has been converted into a bitmap- like TGA or PSD at Full Blend (100%). Then save that AGAIN immediately.

Finally you are ready to see if your machine has the RAM to get Vue to bump it up one one step to 8K x 8K...