f(0) = 4 means we start at value 4.

If the whole wave has period 2, and it is a square wave, then it means for half of the time, the value is (positive) 4 and the other half it is negative 4.

So for the first second, it has value 4, for the second second, the function value is -4.

We write this, using the "rectangular pulse" formula from before:

The graph of this first cycle is: