In a pure e4 application you can not use the old Eclipse 3.x style editors. It is up to you to design how you want your part to work.
You can use the org.eclipse.jface.text
TextViewer
and similar but you can't use anything from org.eclipse.ui.editors
Edit:
You might want to look at this work by Tom Schindl which includes a simple XML editor for e4 although it looks like it may be out of date. Source of this example XML editor can be found here
The Vogella Migration Guide covers migration including the 3.x Bridge code which does allow you to use 3.x code in a e4 part.