The is a critical library for developers working with ASP.NET 4.0 Web Forms who want to add interactive features like calendars, accordions, and modal popups without full page refreshes. While the original library has evolved into newer versions maintained by DevExpress, many legacy projects still specifically require the version compatible with the .NET 4.0 Framework. Where to Download Ajax Control Toolkit 4.0 DLL
You can obtain the necessary DLL files through several official and community channels: AjaxControlToolkit 20.1.0 - NuGet ajax control toolkit 4.0 dll download