I have a MacBook Pro early 2011 - it has been awful with lag in 10.9 betas. The GM was okay, but still lots of lag. The newest beta is working much better for me (not sure if its the beta or the 16g of RAM I just installed - had 4).
I have always had 8GB which I think should be enough for basic use. I'm pretty sure that RAM is not the issue in my situation. Most likely, it's the poor graphic drivers for NVIDIA 320M.
BTW, did you get to observe your macbook on 10.9.2 beta with 4GB RAM? If so, how was it?