Many DNN 3.x modules will run fine in 4.x, but others may not. It depends on if the code used to develop them works the same in the 2.0 framework (which 4.x runs on). For the most part, most modules will work the same but unless they are developed in 4.x they will not be able to be altered without converting them over (for development that is). The skins should work the same as well. Note that any DNN 2.x modules will not work regardless.
To test out, install the modules once the portal is installed, then create a new test tab. Install the modules and then put them on the test tab to find out.