Pressure is force over area. So assuming your cube is at rest and on earth, then the force is mass x gravity and the pressure is mass x gravity / area.
A string quartet is made up of four musicians playing two violins, a viola, and a cello. These are all string instruments played either by plucking the strings or using a bow. The violins are the smallest and typically play the higher notes. The viola and cello are larger instruments and play the lower notes.
If you are thinking the classical interpretation of clarinet quintet, then it is as others have said the clarinet plus the string quartet. However, the all-clarinet clarinet quintet (and not at all unlikely ! ) consists of basically two primary instrumentations: Eb clarinet, 3 Bb Clarinets, and Bass Clarinet or 3 Bb clarinets, Alto Clarinet, and Bass Clarinet. Other combinations are possible but infrequent.