I recommend RAM size based on your PC usage patterns. If you typically only run 1-3 or 4 apps at a time, then 4GB is more than adequate and will generally be quite good. This also, of course, depends on which apps you run. If you're regularly editing video or trying to Photoshop massive pictures, then you might benefit from more RAM. If you only do these activities occasionally, then you'll still be fine.
If you typically like to work with lots of apps running at once, rarely closing any of them, then you should consider more RAM as it will keep the system much more responsive. Again, if your apps are memory hungry, such as video editing, massive Photoshopping, CAD work, etc., then more RAM would also be appropriate.
One thing to keep in mind with newer computers that run Windows and use SSD's is that virtual memory--when Windows starts placing some of the RAM contents onto the permanent storage drive to provide more available memory space--is much more responsive now than when we used to use spinning hard disks, so the overall responsiveness of not having enough actual RAM is much better than it used to be. Virtual memory is still not nearly as fast, even with an SSD, as RAM is, but the penalty is considerably smaller now.
You can monitor your RAM and virtual memory use in Task Manager. This will give you an idea of whether or not you're bumping your head against the ceiling of your current RAM size. If your RAM use is typically above 3GB when you're doing your normal activities on a 4GB system, then you may want to consider a system with more RAM.