That's likely to be impossible I'm afraid. You could possibly "downgrade" a .NET 4.0 application to 3.5, but you need only run into one function call that's not available in 3.5 and you're snookered. Furthermore, VS2008 doesn't support C# 4.0 according to StackOverflow (http://stackoverflow.com/questions/1202289/is-it-possible-to-use-c-4-0-with-visual-studio-2008). That's the new language features, which are somewhat seperate from the framework version (in VS2010 you can use the latest language version with an older framework).
It's worth bearing in mind that .NET 4.0 is a totally new framework, unlike 3.5 which was built on the 2.0 CLR (common language runtime).
Your options are, so far as I can see, stick with your current NopCommerce version or upgrade your IDE and hosting environments.