Some past 'Cat writing on these. AKA "chincello" if you want to Mudcat and/or Google search. I believe Catter LilyFestre had a set. Seems I recall (from the person we both know who told us about these) that they are quite expensive and fragile, but OMG the TONE...... she (that mentor) has a spare fiddle permaently set up that way, which seemed to work well for her.

If you like that deeper tone and don't want to go chincello, Hardi has a 5-string fiddle (a surprisingly nice Chinese). That fifth string allows for the usual 4 violin strings.... and also four viola strings (the ranges overlap). This permits playing in keys many fiddlers find difficult, in addition to the mellow, low tone of playing in the viola range. (PM for details)

~Susan