The Intel HD 4400 graphics chip is already sharing memory with the system RAM since it has no memory of it's own.
Typically on laptops & factory-built desktops, the BIOS is deliberately "cut down" to the very basic settings so you can't screw things up then claim on the warranty to have it fixed for free.
If there is no way to change the shared memory in the BIOS settings then it cannot be changed - - it is fixed at 256 MB