I've juts bought a Dell Inspiron 15 7000 7570
8 GB RAM
GeForce 940MX 4GB GDDR5
Can't see in the system, bios,... 4GB video card memory.
How can i check it
Look in the device manager -- this system has a software-controlled hybrid video setup. The GPU will not show up in the system setup -- it is controlled by Windows drivers, not the firmware.
This isn't a true dedicated GPU - it's part of a hybrid setup.