DW CS4 has a new split code view. Use: View > Split Code from the main menu.
You might not have found it because a button was not added to the toolbar with the Code, Split, and Design buttons for the sake of avoiding confusion. The good news is that the markup to add the button was left in the Configuration/Toolbars/toolbars.xml file, but simply "commented out". So, you can "uncomment" the markup if you'd like that button on your toolbar.
In split view - how to keep the source code view from moving when the lower scennd/frame is adjusted