Thank you Kelly for your tips! I've been able to get my modules converted and running again.

What I did, was:
1. Changed .NET version of my DNN module project to version 4. This resolved all error messages in Visual Studio 2010.
2. Rebuild my solution.
3. Changed for both of my 2 portals in IIS .NET version to 4.
4. I was able to access my 2 portals, but the module gave an error. It had to do with the AjaxControlToolkit.
5. Downloaded the most recent version (40412 - April 12th 2010 Release) of the AjaxControlToolkit and extracted "AjaxControlToolkit.dll" and "AjaxControlToolkit.pdb" (of the .NET 4 version) to the bin directory of my DNN installation (not the one of my DNN module). Make sure the (ACL) file rights on the 2 files are correct.
6. Had to delete and add again in Visual Studio the reference to "AjaxControlToolkit.dll". Rebuild my module. (In another module I had to delete and add references to "DotNetNuke.dll", "DotNetNuke.WebControls.dll", and "DotNetNuke.WebUtility.dll".)
7. My DNN module did not show (on my website) and setting a breakpoint on the page load event handler made clear that it never was invoked. Logging in as Host brought up the following error message:
"DotNetNuke.Services.Exceptions.ModuleLoadException: It is an error to use a section registered as allowDefinition='MachineToApplication' beyond application level. This error can be caused by a virtual directory not being configured as an application in IIS."
Removing the web.config file out of the modules directory solved this error.