Inno Setup: Verify that .NET 4.0 is installed

The InitializeSetup function is called when the Inno Setup executable is run. Inserting this code for a custom script should do what you want:

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'          .NET Framework 1.1
//    'v2.0'          .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
//    'v4.5'          .NET Framework 4.5
//    'v4.5.1'        .NET Framework 4.5.1
//    'v4.5.2'        .NET Framework 4.5.2
//    'v4.6'          .NET Framework 4.6
//    'v4.6.1'        .NET Framework 4.6.1
//    'v4.6.2'        .NET Framework 4.6.2
//    'v4.7'          .NET Framework 4.7
//    'v4.7.1'        .NET Framework 4.7.1
//    'v4.7.2'        .NET Framework 4.7.2
//    'v4.8'          .NET Framework 4.8
//
// service -- Specify any non-negative integer for the required service pack level:
//    0               No service packs required
//    1, 2, etc.      Service pack 1, 2, etc. required
var
    key, versionKey: string;
    install, release, serviceCount, versionRelease: cardinal;
    success: boolean;
begin
    versionKey := version;
    versionRelease := 0;

    // .NET 1.1 and 2.0 embed release number in version key
    if version = 'v1.1' then begin
        versionKey := 'v1.1.4322';
    end else if version = 'v2.0' then begin
        versionKey := 'v2.0.50727';
    end

    // .NET 4.5 and newer install as update to .NET 4.0 Full
    else if Pos('v4.', version) = 1 then begin
        versionKey := 'v4\Full';
        case version of
          'v4.5':   versionRelease := 378389;
          'v4.5.1': versionRelease := 378675; // 378758 on Windows 8 and older
          'v4.5.2': versionRelease := 379893;
          'v4.6':   versionRelease := 393295; // 393297 on Windows 8.1 and older
          'v4.6.1': versionRelease := 394254; // 394271 before Win10 November Update
          'v4.6.2': versionRelease := 394802; // 394806 before Win10 Anniversary Update
          'v4.7':   versionRelease := 460798; // 460805 before Win10 Creators Update
          'v4.7.1': versionRelease := 461308; // 461310 before Win10 Fall Creators Update
          'v4.7.2': versionRelease := 461808; // 461814 before Win10 April 2018 Update
          'v4.8':   versionRelease := 528040; // 528049 before Win10 May 2019 Update
        end;
    end;

    // installation key group for all .NET versions
    key := 'SOFTWARE\Microsoft\NET Framework Setup\NDP\' + versionKey;

    // .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 and newer use value Servicing instead of SP
    if Pos('v4', version) = 1 then begin
        success := success and RegQueryDWordValue(HKLM, key, 'Servicing', serviceCount);
    end else begin
        success := success and RegQueryDWordValue(HKLM, key, 'SP', serviceCount);
    end;

    // .NET 4.5 and newer use additional value Release
    if versionRelease > 0 then begin
        success := success and RegQueryDWordValue(HKLM, key, 'Release', release);
        success := success and (release >= versionRelease);
    end;

    result := success and (install = 1) and (serviceCount >= service);
end;

function InitializeSetup(): Boolean;
begin
    if not IsDotNetDetected('v4.6', 0) then begin
        MsgBox('MyApp requires Microsoft .NET Framework 4.6.'#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;

(Code taken from here: http://www.kynosarges.de/DotNetVersion.html)

First, it checks for the presence of a registry entry that indicates the version of the .NET framework that is installed. If the registry entry is not present, it prompts the user to download the .NET framework. If the user says Yes, it opens the download URL. (You may have to change the version it specifies in the script to version 4.0.)

I also came across [this article on CodeProject][1], which may be a more comprehensive and customizable way of doing what you’re looking for, although it may take more work to understand and will have to be modified to work with version 4.0.