Keep in mind that not only does the operating system use RAM so does the integrated video. 4 GB is a very low amount of RAM considering that your 3581 can have a maximum of 32 GB. I would strongly recommend increasing the RAM total to 16 GB and you can find that here. The performance of the system will be much better with a decent amount of RAM.
Bought Vostro 3581 ,(4gb Ram ,intel i3) just couple of months ago. Very disappointed with the speed of the computer.  The fact is it hangs in opening and even closing apps. It isn't satisfying even minimal need. When I researched a little I was shocked when I viewed the physical memory : Only 446 MB(Always under 600MB) physical memory available out of 4Gb EVEN WHEN NO APPLICATIONS ARE OPEN. Sharing a pic of the same.   [IMAGE]  
Very Low usable RAM