Good question. Are you asking if you can target .NET 4.0 with VS 2008? I don't know, my experience has been that it's like swimming upstream to try to separate the IDE, .NET Framework version, and language version -- best to just get them all.
Are you asking if you can target .NET 4.0 with VS 2008?
Highly unlikely. However, VS2010 will give you a choice of Runtime versions from 4 downwards. VS2008 will give you a choice of Runtime versions from 3.5 downwards.