Answer:
In 2018, the number of people who attended the carnival is:
500 + 5% of 500 = 500 + 0.05 × 500 = 525
In 2019, the number of people who attended the carnival is:
525 - 4% of 525 = 525 - 0.04 × 525 = 504
Therefore, the expression that represents the number of people who attended the carnival in 2019 is 504.