Is WebXtra Compatible with Internet Explorer 4?

No. WebXtra can only create a working browser sprite when Internet Explorer 5, 5.5, 6 or 7 are installed, and only in the Windows platform. Authors can use the member(x).browserServicesAvailable property described in the documentation to check the capabilities of the target machine. This property will return FALSE or 0 when the Xtra is running on the Mac platform or in a Windows machine that does not provide the required services, for example in older machines with IE3 or 4 installed. In these cases the Xtra will still operate without any error messages, but will create just a white sprite instead of a fully operational browser window.