What's so special about 4kb for a buffer length?

That is because 4K is the default cluster size for for disks upto 16TB. So when picking a buffer size it makes sense to allocate the buffer in multiples of the cluster size.

A cluster is the smallest unit of allocation for a file, so if a file contains only 1 byte it will consume 4K of physical disk space. And a file of 5K will result in a 8K allocation.

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Update: Added a code sample for getting the cluster size of a drive

using System;
using System.Runtime.InteropServices;

class Program
{
  [DllImport("kernel32", SetLastError=true)]
  [return: MarshalAs(UnmanagedType.Bool)]
  static extern bool GetDiskFreeSpace(
    string rootPathName,
    out int sectorsPerCluster,
    out int bytesPerSector,
    out int numberOfFreeClusters,
    out int totalNumberOfClusters);

  static void Main(string[] args)
  {
    int sectorsPerCluster;
    int bytesPerSector;
    int numberOfFreeClusters;
    int totalNumberOfClusters;

    if (GetDiskFreeSpace("C:\\", 
          out sectorsPerCluster, 
          out bytesPerSector, 
          out numberOfFreeClusters, 
          out totalNumberOfClusters))
    {        
      Console.WriteLine("Cluster size = {0} bytes", 
        sectorsPerCluster * bytesPerSector);
    }
    else
    {
      Console.WriteLine("GetDiskFreeSpace Failed: {0:x}", 
        Marshal.GetLastWin32Error());
    }

    Console.ReadKey();
  }
}