The .Net Framework is Microsoft's comprehensive and consistent programming model for building applications that have visually stunning user experiences, seamless and secure communication, and the ability to model a range of business processes.
The Microsoft .Net Framework 4 redistribute-able package installs the .Net Framework runtime and associated files that are required to run and develop applications to target the .Net Framework 4.
The .Net Framework 4 works side by side with older Framework versions. Applications that are based on earlier versions of the Framework will continue to run on the version targeted by default.
.Net Framework 4 includes the following new features and improvements :

The Microsoft .Net Framework 4 redistribute-able package installs the .Net Framework runtime and associated files that are required to run and develop applications to target the .Net Framework 4.
The .Net Framework 4 works side by side with older Framework versions. Applications that are based on earlier versions of the Framework will continue to run on the version targeted by default.
.Net Framework 4 includes the following new features and improvements :
- Improvements in CLR and BCL
- Improvements in ADO.Net
- Enhancements to ASP.Net
- Improvements in Windows Presentation Foundation
- Improvements to Windows Workflow Foundation and Windows Communication Foundation