] question , I take it as my own personal note
Look at first answers
It is matter of ServicePoint. Which provides connection management for HTTP connections. The default maximum number of concurrent connections allowed by a ServicePoint object is 2. So if you need to increase it you can use ServicePointManager.DefaultConnectionLimit property. Just check the link in MSDN there you can see a sample. And set the value you need.
Solving:
Just add this code anytime before you begin making the HTTP requests.
System.Net.ServicePointManager.DefaultConnectionLimit = 1000; //or some other number > 4
