1. I recommend:
1. Run a Google search on 'Hemingway's wives' and 'marriages of Hemingway'.
2. Understand Hemingway as a youth, a man, and a writer. This will help you put the pieces together as you determine why he married these woman at important junctures in his life.
2. Research each wife individually:
1. Elizabeth Hadley Richardson - https://en.wikipedia.org/wiki/Hadley_Richardson
2. Pauline Pfeiffer - https://en.wikipedia.org/wiki/Pauline_Pfeiffer
3. Martha Gellhorn - https://en.wikipedia.org/wiki/
4. Mary Welsh - https://en.wikipedia.org/wiki/
3. Recognize that each of Hemingway's four wives played an important part in his creative output. How he related to his wives, and their subsequent influence, is reflected in the tone and mood of his work.
I find Wikipedia.com to be an invaluable research tool. When all else fails, go to Wikipedia.com and gather information from their list of resources.