Why do we use second or first gear while driving up a hillock or ghat section not 3 rd or 4th gear which is supposed to be faster?

it is clear that for driving up a hillock we need to have high torque , but the question is why this high torque is achieved at low rpm so first analyse the graph of torque to speed:

it is clear form the above graph that torque is low at very low and very high speed so why it happens. it happens due to the valve timing as valve are coupled with the crank rotation so at low speed it remains open for a long time which creates losses during compression as a result of which less torque is generated on the other hand if the speed is high enough then valve opens for very less time as a result of which less fuel burns and less power is generated producing less torque.

now another logic which works along is when you change gear piston reciprocates with the same speed it's just the power which is transmitted by the gear from engine to wheel which controls  every things. suppose engine generated power P=torque * rpm so, if arrangement of gear is such that it delivers less speed then definitely it would transmit more power, since gear transmit constant power.