Would not the 4GB help with say a higher resolution panel granting that the user is not a gamer and therefore not needing the GPU itself to be powerful? If they are putting this one out there that perhaps does rendering of some sort but not needing to have an entire game being the end point of what it is being used for? I know some cards have had 4GB in the past and it a lot of that not being used I am sure there are some people who have the opposite problem and might think that card strikes the balance of what they would need in their rig.
The GTX 750 Ti can have 3 x 4K displays connected (not for games obviously). I guess this is where the 4GB card might be of some benefit.
A 4GB card might be good for SLI too, except as commented above it isn't supported.
Considering a GTX 970 would be much faster than 2-way SLI with GTX 750 Ti cards, I can see why they don't bother to support it.
For non-gaming tasks you can get by with a lot less memory. The ONLY theoretical place this might benefit is if video editing programs could properly populate the VRAM and take advantage of it but that's not the case.
I can't think of ANY scenario that more than 2GB makes practical sense for this GPU.
*To be clear, you can increase resolution, anti-aliasing etc for a game to drive up VRAM usage but then that drops the FRAME RATE so low as to be unplayable. That's why there's a relationship between VRAM and GPU performance.