The 4.0 release introduced monumental architectural changes to the Windows app ecosystem:
Offered a streamlined, 41.0 MB lightweight setup option for developers targeting direct client-side operations. net framework 4.0 offline installer download
If you downloaded directly from Microsoft (file is digitally signed), it is a false positive. Some heuristic engines flag old, less common installers as suspicious. Verify the digital signature and submit the file to VirusTotal for confirmation. net framework 4.0 offline installer download