My recommendation is to include ALL the net frameworks in an operating system build now, or at the next update, as it can only be a matter of time before apps appear which require .NET4
You now have a client and full version to choose from, and a 32 bit or 64 bit version as well.
Like the other frameworks, it can be installed with some command line switches, so there is no reason why you cannot include it as a pre-requisite in an EXE compilation of your MSI.
Incidentally, Microsoft KB 318785 has a great list of detection methods for all versions of the .NET framework

I think you misunderstood. The problem is not installing the Framework, it can be installed easily. The problem is packaging applications that are built with .NET 4, because in .NET 4, assemblies should be installed to %windir%\Microsoft.NET\assembly\GAC* instead of %windir%\assembly.

Or maybe .NET 4 assemblies will be installed automagically to the right place? I guess I have to try it.

If I recall correct ;)
The easiest way to add the assembly to the right location you add the file to ex. the default installdir within the project and then select detail on the component holding the file and select .NET and it should install correctly.

Well, that's what we have been doing since day one. And the few times the .NET Framework have changed, it has worked fine, because .NET 2.0 to 3.5 SP1 has been essentially backwards compatible. If you look at the manifest of a .NET 3.5 assembly it says Metadata version: v2.0.50727, e.g. it still pretends to be a .NET 2.0 component. And it is still installed to %windir%\assembly. In theory, a .NET 2.0 source file compiled with the .NET 3.5 Framework can still run on a computer with only .NET 2.0 Framework installed.

.NET 4 has changed all of this. In the manifest it says Metadata version: v4.0.30319. And the file should be installed to %windir%\Microsoft.NET\assembly\. Microsoft has cut off the backwards compatibility.

I created a sample project with Wise. When adding .NET files created with .NET 3.5 it works fine. When trying to add files created with .NET 4.0 I get the following error message:

I can add the file, but no .NET attributes appear in the Assembly tab of the file properties. Thus, the MsiAssemblyname table is not populated and the resulting MSI package is invalid and cannot be installed.

Conclusion. It is not possible to author MSI packages that contains .NET 4.0 assemblies with the current Wise products.

...unless Symantec has plans for adding support for .NET 4 in the near future.

Hot damn!  For the second time this week, I have to start holding my breath. Don't you hold yours, will you?

That's just the obligatory comment. From experience, those breath pauses keep pushing records every time. Now that I think of it, working with these software tools is kind of an apnea experience.