WALTER is a flexible user interface engine for REAPER's track and mixer panels, their masters, the envelope panels and the transport bar. WALTER allows extensive workflow enhancements by offering alternate layouts (globally, or per-track) and theme-based deep customization of many of REAPER's UI elements including their appearance, position, size, colour and how they make the best use of space as panels are resized.
Theme creators and users can use WALTER to radically alter these parts of REAPER's UI. Pre-V4 themes will continue to appear and work as expected, and new themes need not use WALTER. More information is available on the WALTER reference page.
The new prerelease V4 default theme includes a selection of alternate layouts to demonstrate the possibilities. These will be refined and added to during the prerelease period in response to user feedback. A range of stock layouts are also included that will work with most pre-V4 REAPER themes. To change layouts, you can use the menu available via Options->Layouts, or use the screensets window, which has a new tab called “Layouts”, or change layouts for one or more selected tracks via Track ->Set track layout or by right-clicking on a track panel and choosing Set track layout.