Algebra Homework Help

I am working in Eureka Math Algebra 1 module 3 lesson 9 and my teacher gave me this word problem:

Sequences are functions. The domain is the set of all term numbers (which is usually the positive integers), and the range is the set of terms of the sequence. For example, the sequence 1, 4, 9, 16, 25, 36, … of perfect squares is the function:

let : {positive integers} → {perfect squares} Assign each term number to the square of that number.

a. What is (4)? {Type your answer as f(4)= with no spaces}

and i can’t figure out to do it.