Windows generally allocates RAM based what Windows itself needs as well as any programs that is currently running. I think at most Windows can allocate 1.4GB to Intel HD graphics, but that is only if there is 8GB (or more) of RAM installed and if there is enough available free RAM.
4GB is not much to begin with. I have friend who played the Sims 3 on a laptop with only 4GB of RAM at 1366x768 using the Intel HD 4000. He complained that once in a while the game would crash and he did not save the for a few hours. The game was crashing basically because Windows was running of available RAM. I told him to upgrade to 8GB and to save more often. Since upgrading to 8GB of RAM he no longer complained about the game crashing.
Intel HD graphics 620 implies it will be using the integrated graphics from the CPU, the 7th gen kaby lake processor most likely. This will be taken from your system memory (the 4GB of ram), but it usually is dynamic and doesn't stay as a set amount. It will increase or decrease, using more or less ram, depending on what tasks you throw at it.