Editing the template files in IPS4 development is similar to editing the language files.
Template files in Developer Mode are located under /applications/YOURAPP/dev/html/
Admin templates are under /admin and Front End templates are under /front. They're all PHTML files, and are referenced from within the controller like so:
\IPS\Output::i()->output = \IPS\Theme::i()->getTemplate( 'YOUR_APP_HERE' )->TEMPLATE_NAME_HERE( );
We can then import these back into the system when we're ready to release from the Developer Center.