The four parts in which I had to go through this investigation was:

Question

Part 1: Practice
Part 2: Does mass affect the swing rate of a pendulum?
Part 3:Does Arc length affect the swing rate of a pendulum?
Part 4:Does Length affect the swing rate of a pendulum.

It asks why was it necessary to do this investigation in four parts?

I just put, that it was necessary to do this investigation in four parts because each part has a variable which we have to go through to see if it will affect the swing rate of the pendulum.

More info would be appreciated, thanks :-)