The Fourth Five-Year Plan was due to be launched in which year but the Indian government took a 'plan holiday'?

1966

The Fourth Plan was due to start in 1966. By this time, the novelty of planning had declined considerably, and moreover, India was facing an acute economic crisis. The government decided to take a ‘plan holiday’. Though many criticisms emerged both about the process and the priorities of these plans, the foundation of India’s economic development was firmly in place by then.

Thus, the Fourth Five-Year Plan could only get implemented by 1969.
Till now India has completed 12 Five Year Plans (12th was from 2012-17).