macOS Mojave 10.14.2 beta 18C38b
Under normal use the AMD 560X discrete graphics chip gets all 4GB of its VRAM saturated.
AMD 560X 4GB VRAM
information gathered using iStat Menus 6.2
Temperature 113˚F
Core Clock 214 MHz
Memory Clock 300 Mhz
Memory ~3.6GB
Processor ~5%