How do I call async methods in asp.net C# 4.0?

Have a look at using Tasks, this was available in .Net 4 and should help you. A simple example might look like this:

public void MainFlow()
{
   Task taskWork = Task.Factory.StartNew(new Action(DoWork));
   //Do other work
   //Then wait for thread finish
   taskWork.Wait();
}


private void DoWork()
{
   //Do work
}

For more, have a look here