How do I calculate the speedup properly?

Question

I am trying to calculate the speedup of this, but I am getting the wrong answer:

Assuming that we have a program where we have two execution phases. One phase takes 1 time unit to execute and can only execute sequentially. The other phase takes 3 times units to execute sequentially but can be perfectly parallelized. What is the maximum speedup you can achieve?

This is how I have tried to calculate it; I've used the parallelization formula, which states:

$1/((1-P)+P/n))$

Where:

S(n) is the theoretical speedup
P is the fraction of the algorithm that can be made parallel
n is the number of CPU threads

So using the formula in my case:

P is 75% as 3/4 is the total time

I assume n is 1 as nothing is mentioned about it

$1/((1-0,75)+0,75))=1$

What am I doing wrong?