Complete induction: All relevant members or kinds are covered and the property is established for each; the conclusion follows with certainty from sound premises.
Incomplete induction: Some members are known and others are not; the observations may support the conclusion, but cannot entail a universal judgment.
Verifying that all four known rooms are lit supports a certain claim about those four rooms. Seeing lights in selected homes does not justify the same level of certainty about every home in a city.
The decisive difference is coverage relative to the claim, not the raw quantity of evidence.