To disable IPv6 you can create a DWORD 32-bit registry value named “DisabledComponents” in the following registry key branch:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\tcpip6\Parameters\
The value data for DisabledComponents must be set to 000000FF.
This can be done in C# code (
[MSDN] How to: Create a Key In the Registry (Visual C#)
As for converting IPv6 into IPv4. This is impossible. Also one of the biggest issues in switching worldwide from v6 to v4 because basicly the entire world needs to switch at the same time ;)