Sometime after .Net 4 is released it will be available as an optional update on Windows Update. You can configure a machine to test this update process now by electing to receive a beta update. After electing to try the beta update, you'll be offered through Windows Update either the .Net 4 client profile if you have Windows XP, Windows Vista, or Windows 7, or the .Net 4 full profile if you have the corresponding versions of Windows Server. This update will only appear if there are no beta versions of .Net 4 already installed on the machine.
To try the beta update you need to add a registry key by running the following command.
reg.exe add "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\NET Framework Setup\NDP\v4B2WU" /v OptIn /t REG_DWORD /d 1 /f
The beta update will then be available the next time you ask Windows Update to check for updates. This test process will only be available for the next three weeks, through February 8th.