I've never heard this before. Are you using a 3rd party memory controller? If not, do you have loads of apps open? If not then I'd definitely reinstall the OS over the top of your current installation, it would fix any erroneous errors which you seem to be suffering from! That amount of RAM usage just isn't right, even though the game is 64-bit.
"My machine is essentially a fresh install" - Have you wiped it and reinstalled yourself recently to try and resolve this issue? Memory management is the job of the OS.
"I can just not keep sims 4 running for four days." - Why don't you just quit the game? I don't understand why you're keeping it running for 4 days if you're having this memory issue.
I'm not sure how accurate Valgrind is going to be when it hasn't been updated to support Yosemite yet?
Edit - don't forget that you're using a Mac with integrated graphics too, not one with it's own dedicated VRAM.
