I've got a six, and a five. Both Dingwall. The only four strings I've kept are really for sentimental reasons.
There are songs that can be played on a 6 but not a seven? Maybe if the song uses an alternate tuning, but the OP wasn't specific.
There are songs that can be played on a 6 but not a seven? Maybe if the song uses an alternate tuning, but the OP wasn't specific.

