Short answer: how do you define very large scene? 4GB is a lot better than some with either 2 or 3 GB, and most users still have cards in that range. But still a lot less than the 6GB cards, very large or complex scenes tend to be done with users in this group. But the 6GB cards are more expensive.
No easy simple answer, sorry.
Hopefully the following detail will give you more of an understanding so you can relate it to the decision and risk associated with the option of a GTX 970 or some other card.
re is 4GB enough for very large scenes?
Unfortunately that's still a bit of an open question
. By very large scenes do you mean big files, large distances, model complexity, many buildings etc?
Available VRAM is about texturing requirements of a project. Space for textures on the GPU are per uncompressed bitmaps, unless using DDS files. It's possible to do a rough estimate in some cases where you already have the models and main texturing done. The GPU also requires VRAM for other rendering processes, so a 'for textures' VRAM is only part of the test.
VRAM really only becomes an issue when the GPU and Lumion are not able to manage the 'texturing' properly via certain options, and the data gets dropped, so rendering can't be fully accomplished.
More VRAM means more textures can be held in VRAM at one time, ie better for performance.
To put it another way, 6GB gives you 50% more VRAM to play with.
We currently indicate complex scenes be developed with 6GB VRAM cards. We've seen cases where even that is not enough, on it's own. But some adjustments in the scene by using some options, have allowed users to complete 'very large scale' projects.
There are options that can be used in Lumion to manage texture demands for the GPU if needed, so more is always better, but not always required
. For example, there are options such as converting to DDS, reducing texture resolution, simplifying models, reducing some of the effects etc.
If not seen, here's a few recent discussions, more about 980's but that also has 4GB VRAM:
1. todays perfect optimised system for lumion5
2. GTX 980 or Titan GPU?
3. Video Card Options
..and for other details is it better for Lumion work to buy EVGA, MSI, Gigabyte or Asus?
For members to express their view.
What is your price range?
The GTX 980 is another 1000 Passmarks better
It's quite possible to save in some areas of the PC so that other parts can be of a higher specification. For example for the extra for a GTX 980, it might be possible to reduce the CPU but still be fast enough, or less hard-drives, etc etc
-----------------Caveat: these comments are provided only to assist forum members in considering their options for purchasing hardware to use with Lumion. They should in no way be considered as recommendations or qualified advice.