Most people can at least name Groucho Marx. Anybody who's seen a couple of the Marx Brothers movies can name Chico and Harpo. And those who really pay attention can remember Zeppo. But Zeppo was not the original fourth Marx Brother.
Milton Marx, second youngest of the sons, was actually among the first of the brothers to begin performing. He and Groucho were singers in a group called 'The Four Nightengales," and he performed along with his older three brothers as Gummo Marx during their Vaudeville years.
But he became disinterested with performing, and when America entered WWI, he took his draft as an opportunity to leave the act, even though he never actually had to ship off before the war ended.

This script is an alternate reality, wherein Gummo is shipped off to fight, and their mother / manager, insists that Groucho, Chico, and Harpo enlist as well to make sure he returns home safely, so they can keep the act together.

I did a lot of research on the Marx Family, as well as the key players and events of World War I that I wanted to use in my story. I think I found a good angle for each character and their motivations.