Question

why it needs to keep the angle between 4 degree for a simple pendulum?

i didnt able to put the question correctly,but i think you will understand what i want to know.

i have learnt the reason about the question. the answer was if it cross 4 degree then the path of the thing that is hung, does not remain straight line.so why it needs to keep the path straight line?? its my 2nd question.please answer both individually with explanation.