Differences between .NET 4.0 and .NET 4.5 in High level in .NET

What is new in .NET Framework 4.5 & What's new and expected in .NET Framework 4.5

Support for Windows Runtime
Support for Metro Style Applications
Support for Async Programming
Garbage Collector Improvements
Faster ASP.NET Startup
Better Data Access Support
WebSockets Support
Workflow Support - BCL Support

Compare What's New in ASP.NET 4 and Visual Web Developer and What's New in ASP.NET 4.5 and Visual Studio 11 Beta

-Asp.net 4.0
Web.config File Refactoring
Extensible Output Caching
Auto-Start Web Applications
Permanently Redirecting a Page
Shrinking Session State
Expanding the Range of Allowable URLs
Extensible Request Validation
Object Caching and Object Caching Extensibility
Extensible HTML, URL, and HTTP Header Encoding
Performance Monitoring for Individual Applications in a Single Worker Process
Multi-Targeting
.... etc

And Asp.net 4.5 there is also a long list of improvements.
Asynchronously Reading and Writing HTTP Requests and Responses
Improvements to HttpRequest handling
Asynchronously flushing a response
Support for await and Task-Based Asynchronous Modules and Handlers

It breaks down the changes to the framework in the following categories:

• .NET for Metro style Apps
• Portable Class Libraries
• Core New Features and Improvements
• Parallel Computing
• Web
• Networking
• Windows Presentation Foundation (WPF)
• Windows Communication Foundation (WCF)
• Windows Workflow Foundation (WF)

You sound like you are more interested in the Web section as this shows the changes to ASP.NET 4.5. The rest of the changes can be found under the other headings.

You can also see some of the features that were new when the .NET Framework 4.0 was shipped here.

Reference:http://stackoverflow.com/questions/10492647/differences-between-net-4-0-and-net-4-5-in-high-level-in-net