K it's in the green but it says 3.98 gb is being used of the 4gb. Is that good?
Thats fine, I have 8GB and Mavericks will use 7.98GB of it normally. My RAM pressure is also green most of the time.
Now use you Mac as you would normally, load up what you would have running, Safari, Mail, Exchange, Aperture etc etc and see if the RAM pressure goes yellow or even red.
If it goes red then you would benefit from more RAM now, if it goes yellow then you are closer to needing more RAM but not in dire need. If it stays green you won't see any benefit from additional RAM at all at this time.
If it goes yellow you may see some swap used (RAM swapped out to a file on disk), which will slow things down, if it goes red you are likely to be using lots of swap and the machine will likely be very slow to respond. (equally your cpu is likely to be high and fans on from the general high load).
Mavericks keeps stuff it doesn't need in RAM (recently closed apps etc), just in case you decide to reopen them so it can give you a better experience. If however if it
needs the RAM for something else, it will just use it, nothing lost. Keeping your RAM as empty as possible does't bring any benefit at all.