Well, you don't have any applications open so this doesn't tell you much. If all you use is Safari, then, 4GB is plenty.
Basically, the two numbers you need to look at are 'Inactive' and 'Free'. One way OSX manages memory, is it takes recently closed applications and files and LEAVES them in the RAM. They are still there! However they are marked 'inactive'. Then, if you call up that same application, most of it's data will still be in the inactive RAM. The 'free' RAM is RAM that is physically unused. Also, if an application needs more RAM than is available in the free RAM, then it'll use some of the 'inactive' RAM as well. So, basically, Inactive+Free indicated how much RAM you have available.
In order to truly see this, you need to open up the applications you use on a daily basis, a real-world test. For me, that would be Safari with several tabs, Photoshop with a couple images open, and Lightroom in the Develop tab, along with a few finder windows AND Parallels desktop with at least one Windows application open (say MS Excel) All while connected to my external Cinema display (which uses more RAM because I don't have a dedicated GPU on my 13", so it shares memory with the system). That would be a realistic example of a worst-case-scneario. I obviously can't use all of those applications at the same time, but it's not at all unrealistic for me to have all of those things opened when I'm actually getting some work done. Obviously, I could just fire up as many VM's as I can and max out my RAM, but that wouldn't be realistic, since I never do that! Just come up with a scenario that stresses the machine in a realistic scenario you might actually be in, and see where you are at then.
THEN, you need to look at the 'page outs' number. This is going to be the kicker. That needs to always be 0. If it's not 0, you don't have enough RAM. Page outs are when there is not enough RAM for the system to allocate, so it starts to use your hard drive. Even the fastest SSD's on the planet are several times slower than RAM, in both read/write and the very important access times. So, your system WILL slow down when it is paging out. So start working up. With your web browser open, you have no page outs. So, for browsing the web, you don't need more RAM. Open up a few other programs you use, maybe a game.. any page outs? Escalate all the way up to the most radical-yet-realistic example you can think of (like mine above, for me), and take note of where your page outs are. If you are using 10 gigs of virtual memory (page outs) and your full 4 gigs of physical memory, then you need 16 gigs!
Ultimately though, I think budget decides. Aside from cost, there is NO advantage to LESS RAM. 16GB can only benefit you. 8GB is probably sufficient, but there's no advantage at all to 8GB over 16GB aside from cost. So take a look at your pageouts, determine how much performance you need, and determine your budget.
-John
