HI. It appears that you may not have enough memory. Your graphics system actually uses 1536 MB (or about 1.5 GB) of your 4 GB of memory as VRAM. This is large piece of your existing memory. You are in luck because yours is one of the last models where the memory is not soldered to the mother board. Apple claims that the max. RAM for your machine is two sticks of 4 MB memory. Very reliable sources note that your processor should be able to take more: 16 GB (Actual) 8 GB (Apple). Thus you could obtain 2 sticks of 8 GB RAM and place them in your machine. If you go to Apple they will only install 8 GB max of RAM.
If you go to a place like http://www.crucial.com/
or another trustworthy site (not Amazon), they will tell you what will work and will stand behind their product (which is normally much less expensive than Apple's). They will also tell you how to install it (or whatever friend you have do for you). Apple also has a site for instructions on installing RAM in your machine: https://support.apple.com/en-us/HT201... I do not know if this will work for you, but I know several people that had good luck with this method. It would have been better if you had purchase a Mac with a separate graphics card along with the Intel system. These cards come with their own VRAM and the capability to switch between whichever graphics was needed at the moment. Even with an extra processor you would most likely have had to pump up the memory. Four GB does not go very far today. Good luck.