System memory that is shared for vram is dynamically allocated, so it's not a fixed size. Depending on application, actual use will vary depending on graphics / resolution settings. So potentially 4GB but may only use a portion of it and rest would allocate to where it's needed.
You can set a fixed size in the Bios but wouldn't bother. Igpu will use as much ram for video when needed. Mind you, 4GB worth of stored texture size in a game generally means a heightened level of details, which means performance beyond an igpu is capable of.
Adding another stick (for dual channel?) is worth doing anyway for better overall performance.