The meaning of arguments of is_primitive

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Say we have

R.<x> = PolynomialRing( GF(5) )
f = x^2 + 2*x +3

then we can use f.is_primitive() to tell if f is primitive. But is_primitive has two argument n and n_prime_divs. The document say

"n" (default: "None") - if provided, should equal q-1 where
        "self.parent()" is the field with q elements;  otherwise it
        will be computed.

But this is whose 'self'? Is it the self of polynomial f? so n=4? I think here the n shall be 5^2-1. The following code will result in error.

f.is_primitive(4, [2])

but

f.is_primitive(5^2-1, prime_divisors(5^2-1))

will give the correct answer.