Why ASP.NET 4.0

• Delivers Performance and Scalability: By using new feature of ASP.NET 4.0 we can quickly and confidently manage large amount of data. We can also easily manage increasing volumes of traffic.
• Faster Response Times:  Caching and Session State Compression helps enormously to improve the response time. 
• Through Output-Cache Providers cache content can be stored to any persistence mechanism such as databases, disks, cloud storage and distributed cache engines.
•  With Session State Compression feature, developers can often reduce the time it takes for a web application to respond by reducing the size of session data.
• Reduces Code, Cuts Costs, and Accelerates Development: With Visual Studio 2010 & .NET Framework 4, we minimized the application code and the need for additional resources.  Instance: Interfaces for modelling and editing information in any database can be easily created.
• Boosts Customer Satisfaction: Customer Satisfaction can be increased by reducing the errors in production code through Compile-time checking, integrated development and test environment capabilities. 
• Promotes Visibility:  Now Web page can be designed to increase its global visibility through Search Engine Optimization (SEO). 
• Now we can ensure that every action that is a page has a proper “title tag, Meta description, and canonical URL”.This save the requirement of adding these manually or from writing own code to create Meta tags. This property can be set at run time.
• Multiple URLs can be created for a single page that will help developers to move pages and update URLs without breaking any in-bound links from other sites.
• Search engines and other user agents that recognize permanent redirects will store the new URL that is associated with the content, which eliminates the unnecessary round trip made by the browser for temporary redirects.
• More descriptive/meaningful URLs can be provided that can be easily understandable by users and search engines
• Routing in ASP.Net 4 provides full class support by allowing users to define any custom route to a web form page
• Larger amount of information could be sent over by a longer URL character length.
• Web.Config is less cluttered ,clean and simple.
• ClientIDMode property lets you specify more precisely how the client ID is generated for controls. 
• Client Id can be generated for each and every control through ClientIDMode property
• Provides a controlled approach for starting up an application pool
• Reducing unnecessary subsequent request to the server
• Easily build data driven (asynchronous) application.
• Intentionally relax XSS checks for specific pages or for specific types of requests.
• Now multiple ASP.Net application can run in a single worker process.
• Now administrators have a more granular view into the resource consumption of individual applications running in a single worker process.
• Multi-targeting allows you to use and target multiple versions of .NET.