(i) Using Newton's Second Law, the equation of motion up the hill is:

\(DF - 1550 - 1400g \sin 4^{\circ} = 1400 \times 0.4\)

Solving for the driving force \(DF\):

\(DF = 3086.59 \ldots \text{ N}\)

Using the power formula \(P = Fv\), where \(P = 30000 \text{ W}\):

\(30000 = (1400 \times 0.4 + 1550 + 1400g \sin 4^{\circ})v\)

Solving for \(v\):

\(v = 9.72 \text{ m s}^{-1}\)

(ii) For constant speed, the net force is zero:

\(DF - 1550 - 1400g \sin 4^{\circ} = 0\)

Solving for \(DF\):

\(DF = 2526.59 \ldots \text{ N}\)

Using the power formula \(P = Fv\) with \(v = 40 \text{ m s}^{-1}\):

\(P_{\text{max}} = (1550 + 1400g \sin 4^{\circ}) \times 40\)

\(P = 101063.6 \ldots \text{ W}\)

Thus, the maximum possible power is 101 kW.