I'm having the same issue with my MB Pro retina witch as 8 gigs of RAM

Try that :

Reduce your rendering resolution 2000 x 2000 is a lot for a video card witch has to cope with a Retina display, try 1200 x .... And adjust antiAliasing.max to 1 instead of 2 if it's a very high res rendering

Before saving, close all other programs and virtual desktop, they takes a of video memory.

If you open terminal, clean the memory with Sudo purge command in the terminal.

You could also reduce screen font size to bigger font under System preference and Monitor, when rendering is finished, go back to normal setting.

NVRAM Reset : Reboot and hold the Option + Command R + P to clean video memory, follows instructions here : https://www.youtube.com/watch?v=rQ__4qXb0NQ

Practice all these tricks and see what is working best for U

Good Luck