Why does OSX use 6GB of RAM after restart?
Memory being used doesn’t mean the memory can’t be used for anything else. Mavericks will put idle memory to work if it can.

For instance, 2.5G is being used for file cache — storing files in memory so it doesn’t have to hit the disk for them. As you consume more memory, that will go away.

Now, if it crashed on you due to swap pressure, you may not have enough memory for the work you’re doing with the machine (Or maybe not enough free disk.), but there’s no indication you have a problem right away.

I have 4GB on a non-retina MBP. It’s been up for four days, I run a fair amount of crap. It’s using 3.96GB of that memory, and hasn’t touched swap at all. (It’s also compressed about a gig of memory.)