I don't have a copy of DNN 4 handy but look for manageusers.ascx. That control does a number of different functions depending on what the situation is but in DNN 5 it handles the user registration.

Well, that's what i thought at first. I searched all the controls in the users and profile available in the admin folder including the manageusers control but wasn't able to locate the registration control. The control is referenced in the user control as propertyeditorcontrol :
<dnn:propertyeditorcontrol id="UserEditor" runat="Server"
    enableClientValidation = "true"
    sortmode="SortOrderAttribute"
    labelstyle-cssclass="SubHead"
    helpstyle-cssclass="Help"
    editcontrolstyle-cssclass="NormalTextBox"
    labelwidth="150px"
    editcontrolwidth="200px"
    width="350px"
    editmode="Edit"
    errorstyle-cssclass="NormalRed" DisplayMode="Table"/>

The propertyeditorcontrol cannot be located anywhere in the set of controls available in DNN.
It's a shame though, this should be a quite easy task to do and maybe an option in the core to register either using a username or an email. I hope you guys include it, in the version 6 of DNN which i am pretty excited about, since the core will be clearer to C# developers.
Thanks for you time.