Concept:
Fertility levels are used in demography to study population replacement and growth.
Step 1:
Fertility level refers to the number of children born in a population.
Step 2:
Replacement level fertility means parents produce only enough children to replace themselves in the next generation.
Step 3:
At replacement level fertility, population size remains stable over time if migration is ignored.
Step 4:
Terms such as surplus level, family level, and progression level are not the correct demographic concepts for population replacement.
Step 5:
Therefore, the correct answer is Replacement level.
\[
\boxed{\text{(4) Replacement level}}
\]