I had this issue when I had added telerik:RadScriptManager to the .master file of my custom template. As it turns out this doesn't need to be added as Sitefinity adds a script manager as required (I think this might be what causes the issue in the back end, script manager being added twice) If this is your case, you can remove the RadScriptManager but be sure to add an extra attribute to any custom user controls that use RadControls (or other) that require a script manager. The new attribute (that wasn't around in SF3.x) is:
public partial class CustomMenu : System.Web.UI.UserControl
// menu code
If that is indeed the cause of your issue, hopefully it will help until the next build is available.