This is now implemented in SVN as a 'Tabs and Spaces' indent type preference and per-document setting, plus a Tab Width pref (the old pref is now called Indent Width). The main editing actions should work as expected in the new mode, but some of Geany's code insertion features (e.g. default file templates) might still use tabs. Feel free to file bugs/requests to improve things further (or post here).
How can I set "display-tab-width" to 8 spaces and "indent-width" to 4 spaces using current editor preferences dialog? I couldn't do that using latest SVN version of geany.
Still there is no distinguish between display tabs width and single indent width. At least I still can't set up the required configuration....