You will compare and contrast just like you have done in prior essays. The difference will be that you are using four beginning points, or photographers in this case, instead of two. The process will be the same, but with input from more sources.
For example, if you were comparing the work of Ansel Adams, Margaret Bourke-White, Matthew Brady, and George Eastman, you could point out that all four worked with black-and-white photography. All were pioneers in their particular aspects of photographic work, and contemporary photographers owe a great deal to all four. Contrasts between their chosen specialty areas, the processes they used to compose their pictures, and the techniques involved in developing the pictures they took could illustrate the development of photography from a very specialized and labor-intensive process to an activity that became popular and easily available to everyone with an interest.