Could someone help me fix an error?

[Preparation]

------ Build started: Project: TBC Launcher, Configuration: Debug Any CPU ------
Project file contains ToolsVersion="4.0", which is not supported by this version of MSBuild. Treating the project as if it had ToolsVersion="3.5".
C:\Windows\Microsoft.NET\Framework\v3.5\Csc.exe /noconfig /nowarn:1701,1702 /platform:AnyCPU /errorreportrompt /warn:4 /defineEBUG;TRACE /reference:"D:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\v3.5\System.Core.dll" /reference:"D:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\v3.5\System.Data.DataSetExtensions.dll" /reference:C:\Windows\assembly\GAC_32\System.Data\2.0.0.0__b77a5c561934e089\Syste m.Data.dll /reference:C:\Windows\assembly\GAC_MSIL\System.Deployment\2.0.0.0__b03f5f7f11d50a 3a\System.Deployment.dll /reference:C:\Windows\assembly\GAC_MSIL\System\2.0.0.0__b77a5c561934e089\System.d ll /reference:C:\Windows\assembly\GAC_MSIL\System.Drawing\2.0.0.0__b03f5f7f11d50a3a\ System.Drawing.dll /reference:C:\Windows\assembly\GAC_MSIL\System.Windows.Forms\2.0.0.0__b77a5c56193 4e089\System.Windows.Forms.dll /reference:C:\Windows\assembly\GAC_MSIL\System.Xml\2.0.0.0__b77a5c561934e089\Syst em.Xml.dll /reference:"D:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\v3.5\System.Xml.Linq.dll" /debug+ /debug:full /filealign:512 /optimize- /out:"obj\Debug\TBC Launcher.exe" /resourcebj\Debug\TBC_Launcher.TBC_Launcher.resources /resourcebj\Debug\TBC_Launcher.Properties.Resources.resources /target:winexe TBCLauncher.cs TBCLauncher.Designer.cs Program.cs Properties\AssemblyInfo.cs Properties\Resources.Designer.cs Properties\Settings.Designer.cs
D:\Users\Glare\Desktop\llauncher\TBC Launcher\TBC Launcher\TBCLauncher.cs(17,24): error CS0234: The type or namespace name 'Tasks' does not exist in the namespace 'System.Threading' (are you missing an assembly reference?)
D:\Users\Glare\Desktop\llauncher\TBC Launcher\TBC Launcher\Program.cs(4,24): error CS0234: The type or namespace name 'Tasks' does not exist in the namespace 'System.Threading' (are you missing an assembly reference?)

Compile complete -- 2 errors, 0 warnings
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

I believe I need 4.0 references? But I only have up to 3.5