User347430248 posted
Hi kengkit,
You had asked,"can i open my web application which build under framework 4.0 in visual studio 2012?"
Yes, You can open your Web Application Project in Visual Studio 2012.
Visual Studio will find the version of framework by itself.
You can also go to Project properties and change to the desired version of framework which is currently available in Visual Studio 2012.

Regards
Deepak