I got a Strix GTX 970 (Asus) and it should be too 3,5 + 0,5 GB Vram (4GB total Vram) .
I have tested it with Manu games as GTA V all ultra until it was using Vram between 1-4080MB it was working fine (in OC) so there was not any flooding of performance using over 3,5 GB. I don't really know well what should this 3,5 +0,5 GB issue however for me it works fine.
Btw soon I'll sell my GTX 970 to get a 1060 , if you want to buy it contact me xD.