Almost correct. You need to change your r vector to get the correct time vector, because you only want to plot the sine wave for 4 seconds.
r = linspace(0,stopTime,stopSample);
linspace creates a vector with evenly spaced points between 0 and stopTime. stopSample is the number of points. The plotted data is now not even a fourth of a full sine wave which makes sense, because your frequency is 0.05 Hz ( pi/10/(2*pi) ) which makes a full sine wave take 20 seconds.