A Pipa is a traditional Chinese plucked instrument, also called a Chinese lute.  It reached Japan in the 8th century AD  and there was renamed the biwa.  I believe the body is Rosy Sandalwood and the top is quality Paulownia.  The tuning is  C - F - G - C, with the top note around Middle C.   A truly unique and beautiful instrument, this one has 31 frets, 

4 String Japanese Pipa