If I have an application built to target .net 3.5, will it run if all the user has installed is .net 4.0? A link to official msdn documentation stating this would put my wor
Yes, it can. You can say that you support .NET 4 in your app.config file:
Note that changes in the framework may break your app. For details, see MSDN's list of .NET 4 migration issues, which includes recommended changes.
If you still want to support users with only .NET 3.5 installed, you also need to add a element:
Note that the above configuration prefers .NET 4.0 if that is installed. Changing the order above makes the application prefer .NET 3.5 if that is installed. A complete table of possible configurations and effects can be found in MSDN's how-to on configuring your app to support .NET 4.0 or 4.5.