ajaxtoolkit issue in asp.net

Warning 3   The following assembly has dependencies on a version of the .NET Framework that is higher than the target and might not load correctly during runtime causing a failure: AjaxControlToolkit, Version=4.1.7.429, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e. The dependencies are: System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a; System.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a; System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35; System.Web.Extensions.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35; System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089; mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089; System.Configuration, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a; System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089; System.Xml, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089; System.Drawing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a; System.Data, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089; System.Xml.Linq, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089; System.Core, Vers.... You should either ensure that the dependent assembly is correct for the target framework, or ensure that the target framework you are addressing is that of the dependent assembly.