For Expression Web 3 the settings for each web site are located at
%Appdata%\Microsoft\Expression\Web 3\SiteData
and for Expression Web 4 the settings are at
%Appdata%\Microsoft\Expression\Web 4\SiteData
Each website has its own folder at these locations, which contains an XML file named user.config. To move the settings for a website from Expression Web 3 for use in Expression Web 4, copy this folder (and its contents) to the same location for Expression Web 4.
To find the folder in Windows Explorer, type the location into the Windows Explorer address bar, the %Appdata% will be translated to the correct place for the operating system used (Windows XP, Vista, Windows 7, etc.)
With the website folders open in Windows Explorer, scroll through the Folders bar and use the + symbols to open the folders for Web 4 until the SiteData folder is revealed. Then right-click and drag the required SiteData folders from Web 3 to Web 4\SiteData. Choose Copy from the context menu when the mouse button is released, or the files will be moved and then be unavailable to Expression Web 3.