4 strings? My old Bulbul had eight... The one in the diagram here: http://chandrakantha.com/articles/indian_music/bulbul.html has six.

Waves of Nightingales? I have my doubts! This was one of the few instruments that I was actually glad to see the back of, although I've heard some astonishing Bulbul music since. Like this: http://www.youtube.com/watch?v=L8So6q3bt8Q