We can first consider the number of ways the two people who sit together can be chosen. There are 4 ways to choose the pair of people who sit together (i.e., person 1 and 2, person 2 and 3, person 3 and 4, or person 1 and 4).
Once we have chosen the pair of people who sit together, we can treat them as a single entity, reducing the problem to arranging three entities (two individuals and one pair) in a row. There are 3! = 6 ways to arrange these three entities.
Finally, the pair of people who sit together can be arranged in two ways (i.e., left to right or right to left) within their pair.
Therefore, the total number of different ways four people can sit in a row at the opera if two of them sit together is 4 x 6 x 2 = 48.