I have a Mac OS X with 8 GB 1600 MHz DDR3 RAM and when I start unreal engine it complaints about Low RAM and asks that for best performance I should have at least 8 GB of RAM.
I'm not sure why is this is case, is it possible that its getting a lower share of RAM or something similar?
The Unreal Editor version is 4.7.5
Edit: The processor is 2.3 GHz Intel Core i7, and is 64 bit.
Edit: The graphic processor is Intel Iris Pro 1024 MB.
Why would UE4 not be able to access all of your RAM? I do not consider that case very likely.
Also, I would not worry about the RAM usage of UE4 too much. I do a lot of work with the engine and it rarely uses more than 3GB of RAM. Just make sure that your system as a whole has enough RAM for the running processes to prevent swapping.
Anyway, the bottleneck in your system is probably the graphics processor, so if your engine runs too slowly you should reduce the performance setting inside the engine.