Re; swap/paging files - It's always better to add physical memory rather than trying to emulate memory with a file located on a HDD/SDD. I cannot stress this enough, especially for mechanical drives (HDD).
Rather than playing with swap/paging file settings, get some more ram in your server.
In addition, while
Windows Server 2012 minimum recommended RAM is only 512MB, you're probably aware that windows runs quite poor on that little amount of RAM. Simple tasks take forever to complete (ie; loading IE and navigating to Microsoft.com).
To expand one what James/Gizmo previously wrote -
Are you really running your site with only 4gb ram? That's the same amount I have for my entire VPS, and my sites aren't nearly as active as yours...
At this point, since it's more than likely a server resource issue, and that's outside the scope of user to user support on the forums, I'd advise contacting your host/data center regarding server tuning and recommendations on their end.
When selecting the system specifications for a server solution, even the optimal system requirements recommendations from Microsoft might not suffice. It is a best practice to assess the server specifications of the planned server role while taking the load during the time of deployment and future growth into consideration. For example, a Windows Server 2012 system running the Windows Server 2012 Essentials will require 8GB of RAM to run adequately. Therefore, size the system accordingly and test the load before going live into production.
From what I know already, you have the most current versions of Apache, SQL, PHP and probably a few other services constantly running along side the base Windows Server services. And now you're trying to have over 200 concurrent connections (your users) all performing 10 to 1,000+ database requests at the same time, during your regular usage hours, on a Windows Server 2012 setup with only 4GB of RAM.
