GTX 970 - Blender278 - CUDA Error Hi,

I've been using Blender 2.78a on Windows 10 Pro. The problems I'm trying to solve and I search for many solutions around the www is the classical Memory CUDA problems. I use a GTX 970 and batch render as you can see on the image. It has 4 Gb VRam and I use less than 2 Gb for the render...
Seems Cycles can generate the first bucket but when it goes to the second always crashes. I don't understand anything and it's so frustrating for me...
Things I've done:
- http://artificialflight.org/blog/2013/cycles-crash-cuda-tdr-error/
I put 64, 16, 256...
- I have last drivers
- Reduce bucket size

Could anyone help please? Thanks in advance
Searching a little bit more and more, I find many things about Mem Cycles consume and real consume. With GPU-Z I can see a real read.
First of all, the modifiers turned on in render, Subdiv mainly. When blender Mem says 1670Mb, is the limit for my 4Gb GTX 970.
All textures is better to be JPG, PNG. Only use float as EXR or HDRI on particular cases like environment lighting.

Maybe, one day, Cycles developers would think using GPU only to process and hardware RAM to cache everything, like Redshift and Octane do :)