function IsDotNetDetected(version: string; service: cardinal): boolean;
// Indicates whether the specified version and service pack of the .NET Framework is installed.
//
// version -- Specify one of these strings for the required .NET Framework version:
//    'v1.1.4322'     .NET Framework 1.1
//    'v2.0.50727'    .NET Framework 2.0
//    'v3.0'          .NET Framework 3.0
//    'v3.5'          .NET Framework 3.5
//    'v4\Client'     .NET Framework 4.0 Client Profile
//    'v4\Full'       .NET Framework 4.0 Full Installation
//
// service -- Specify any non-negative integer for the required service pack level:
//    0               No service packs required
//    1,2,etc.      Service pack 1,etc. required
var
    key: string;
    install,serviceCount: cardinal;
    success: boolean;
begin
    key := 'SOFTWARE\Microsoft\NET Framework Setup\NDP\' + version;
    // .NET 3.0 uses value InstallSuccess in subkey Setup
    if Pos('v3.0',version) = 1 then begin
        success := RegQueryDWordValue(HKLM,key + '\Setup','InstallSuccess',install);
    end else begin
        success := RegQueryDWordValue(HKLM,key,'Install',install);
    end;
    // .NET 4.0 uses value Servicing instead of SP
    if Pos('v4',version) = 1 then begin
        success := success and RegQueryDWordValue(HKLM,'Servicing',serviceCount);
    end else begin
        success := success and RegQueryDWordValue(HKLM,'SP',serviceCount);
    end;
    result := success and (install = 1) and (serviceCount >= service);
end;

function InitializeSetup(): Boolean;
begin
    if not IsDotNetDetected('v4\Client',0) then begin
        MsgBox('MyApp requires Microsoft .NET Framework 4.0 Client Profile.'#13#13
            'Please use Windows Update to install this version,'#13
            'and then re-run the MyApp setup program.',mbInformation,MB_OK);
        result := false;
    end else
        result := true;
end;

(代码摘自：http://www.kynosarges.de/DotNetVersion.html)

首先，它检查是否存在指示已安装的.NET框架版本的注册表项。如果注册表项不存在，它会提示用户下载.NET框架。如果用户说“是”，则会打开下载URL。 (您可能必须将其在脚本中指定的版本更改为版本4.0。)

我也遇到了this article on CodeProject，这可能是一个更全面和可定制的方式来做你正在寻找，虽然可能需要更多的工作要了解，将必须修改与4.0版本的工作。