Gpu not using full vram

[Redbeardthebearslayer]

I just build my first pc and I have been running some benchmark tests to see how everything is running. I have been a little let down by my firestorm and heaven score. I have done some checking with Cpu-z and it is saying that my Gpu only has 4gb of memory which should not be correct. If I go to screen resolution, click advanced settings, it shows my gpu has 8gb of vram.

Cpu Intel i7 5930K 3.5 6-core
Motherboard EVGA Classified x99
Memory Corsair Vengeance LPX 16gb (4x4)
SSD samsung m.2 128gb
HDD western digital wd green 2TB
Video card Asus radeon R9 390 8gb
Power supply corsair 1000w 80+ platinum

How can I check to see how much vram my gpu is using?
If it is capped at something less than 8gb how do i raise it?

This is what I get if I look under advance setting under screen resolution
But this is what it tell me my vram is under Cpu-Z

 

[kanewolf]

Your GPU will use as much as it needs. If you are at low resolution, 1080p, then less VRAM is required compared to 4K or multiple 1440p displays. AntiAliasing can use more VRAM depending on the settings.