Actually, that is completely untrue. Since the impulse of a rocket motor is (for a given propellant type) directly related the amount of propellant you can fit in the motor case (volume), the diameter of the motor scales on a second power while the length scales on a linear power.
(Volume= Pi / 4 * D^2 * L )
Also, a 40,000 Ns motor is damn big for 4" design. Most amatuer manufacturers (Cesaroni, Aerotech, etc...) max out at 20,000 Ns for a 4" diameter. Not to be picky either, but for 5.1 seconds, 40,960 N s = 1805 lbf.
Additionally, too slender of a motor messes up the gas dynamics of how the propellant burns and escapes through the nozzle (assuming is a core burning propellant).
But just talking about the rocket itself, which is what I was referring to as slender. But hey, if it works it works, I was just going by some rules of thumbs!
Nonetheless, that is a damn nice job of the CF body!