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Maybe you want to use tinymce's compat3x plugin to load the legacy code.
General
General: Contenido itself implements a few own plugins, for instance the close-Plugin and the confullscreen-Plugin inside con_tiny.js inside the tinymce 4 folder. These can be seen as small examples of how a basic plugin can look like. For more complex examples you can download the tinymce 4 development package and check out the non-minified plugins from the creators of tinymce.
Make sure to use tinymce 4's windowManager in order to display model dialogs. Pass custom parameters like in their example to the new window. Their documentation mentions the setParams function to write into the variables you passed to the dialog during opening. Close the dialog window using the tinymce instance in the calling window, e.g. using "parent.tinymce.activeEditor.windowManager.close(window)". Bind the dialog instance you created using the open function of the windowManager using the on function of the instance to the close event.
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