I did some investigating on one or two of the the web's interactive tuning/chord sites but will report if i find anything else that's interesting, one interactive thingy here for Ukes, Mandolins and Banjos could be useful...

"All chords are calculated and therefore may not be really playable!"

http://4stringchords.com/

and Alan's open D chart ...

https://storage.ning.com/topology/rest/1.0/file/get/305749808?profile=original

and a tuning guide for Banjos with sound bites...

http://www.get-tuned.com/online_mandolin_tuner.php

For starters here's a  GDGB chart i found on the Handmade Music Clubhouse submitted by Billy Freedom, lots here to try out!

...and a link to a new 4 string chord chart by Greasestains...

http://www.cigarboxnation.com/group/greasy-s-easy-chords/forum/topics/open-g-chords-four-4-strings-tuned-to-g-d-g-b-or-d-g-b-d