Concept:
Population changes depend on births, deaths, immigration, and emigration.
Step 1:
When the number of people entering and being born equals the number of people dying and leaving, population size remains stable.
Step 2:
Zero population growth occurs when:
\[
\text{Births + Immigrants = Deaths + Emigrants}
\]
Step 3:
• Population growth rate measures increase or decrease in population.
• Morbidity rate refers to disease occurrence.
• Total fertility rate measures average births per woman.
Step 4:
Therefore, the correct answer is Zero population growth.
\[
\boxed{\text{(4) Zero population growth}}
\]