I tried lots of things but there seems to be no way to use a variable product version string (there's a number of threads on the net as well that come to this conclusion). Moreover in our project we also use the version string in the shortcut names, the setup file name etc and found no way to store this in a single place. We found a very manageable solution though via automation. Here's a sample that updates version number and filename using VBScript, but you can also use C# for instance.
Set ISWIProject = CreateObject("IswiAuto19.ISWiProject")
ISWIProject.OpenProject "Our.ism"
ISWIProject.ProductVersion = "4.0.2.0"
ISWIProject.ISWiProductConfigs.Item("Release").SetupFileName = "App_4.0.2.0_Setup"