Solve the equation \( \frac{\partial u}{\partial t}=\frac{\partial^{2} u}{\partial x^{2}} \) subject to the conditions \( u(0, t)= \) \( u(1, t)=0 \) and \( u(x, 0)=\sin \pi x \) using Crank - Nicolson method. Use \( h=0.25 \) and \( k=1 \). Find \( u(x, t) \) for \( 0 \leq x \leq 4 \) and \( 0 \leq t \leq 1 \).