Slow Computer Question

I use a Mid-2011 iMac 21.5" (with 4 users on it).

I have noticed that the computer is runs quite slow when I am the only one logged in. The only applications I typically have open at once are Xcode, Safari and occasionally some other applications such as Reminders and notes.

When I checked Activity monitor to see how much RAM it was using, I noticed it said it had used 3.97-3.99 out of the 4 GB of ram that it has (See Screenshot).

here is the cpu graph:

I also ran EtreCheck as requested:

http://ift.tt/1787tnT

I am considering buying extra RAM but I wanted to make sure that this was actually the cause of the problem before investing in $100 RAM chips.

Would upgrading the RAM from 4 GB to maybe 8 or 16 GB fix the issue? If not, what would fix the issue? Is there anything else that needs fixing?