How to correctly interpret the $\ln(n)$ approximation ratio of the set cover problem under its integer formulation context?

The wikipedia article of the set cover problem stated the following point regarding the inapproximability of the greedy method "When $$n$$ refers to the size of the universe.... it cannot be approximated to $$\left( {1 - o\left( 1 \right)} \right)\ln \left( n \right)$$ unless P = NP". Now suppose that I have the toy example (integer programming formulation) of a simple set cover problem

$$\begin{array}{*{20}{c}} {\mathop {\min }\limits_{{x_i} \in \left\{ {0,1} \right\}} }&{{x_1} + {x_2} + {x_3} + {x_4}}\\ {}&{{x_1} + {x_2} \ge 1}\\ {}&{{x_2} + {x_3} \ge 1}\\ {}&{{x_1} + {x_3} + {x_4} \ge 1}\\ {}&{{x_1} + {x_4} \ge 1}\\ {}&{{x_2} + {x_3} + {x_4} \ge 1} \end{array}$$

Does this mean that $$n$$ from the wikipedia article refers to the number of decision variables (4 in this case) or the number of constraints (5 in this case). Particularly, I want to know if $$\ln \left( n \right)$$ refers to the number of decision variables or the number of constraints when talking about the approximation ratio?

Would you kindly help me with this?

$$n$$ is the number of elements. Elements are what needs to be covered; therefore, the constraints.
• So in my question $n=2$ right ? Commented Jul 23 at 15:13
• @TuongNguyenMinh $n = 5$ in your example Commented Jul 24 at 10:41