I think I'd be stuck just catching all four of them and releasing the ones that weren't shiny..
Just a note, Horde Battle capturing works like Double Battle capturing - you can't throw Pokeballs when there are more than one Pokemon on the other side. This means even if you run into a horde of 5 Shiny Pokemon, you can only capture one of them (sadface).
And for everyone trying to determine which size Pumpkaboo/Gourgeist they have, I recommend spending a few minutes capturing a few. I went out and captured about 12 of them and I got at least one of each size. I arranged them from smallest to largest in my PC box and when I'm not sure about the size I just stick it under one of the columns and repeatedly press up and down to compare sizes, switching columns until they match.