Wow, this was brilliant! I loved everything about it. Absolutely captivating plot, excellent characterisations. I've somehow never read Arthur Conan Doyle but I have a feeling you got Dr. Watson's voice spot on! I felt transported to that time period myself.
I can't even begin to tell you how much I enjoyed seeing Snape, Harry and Draco (I so loved that you had Draco there, too!) meeting Holmes and Watson, and all five of them interacting with each other. And things like each party's reactions to the Muggle and wizarding world respectively, or the differences between the two time periods. I loved that kind of details. And I so loved how you wrote everyone! Especially Harry, Snape and Draco, since I'm obviously way more familiar with them - Snape was so hot, acting like the powerful wizard he is. *g* All your original characters were wonderful, too.
Getting back to the plot - the fourth Hallow was a brilliant idea and I loved how you had things like the past Malfoys that tied so well with the HP canon of the present time period.
I found myself wishing Holmes had a more major role, but I suppose you couldn't have done that without taking the focus from our three wizards. Which makes me think I would gladly read another 56k and another and another and another from each of the other four characters' POV if they existed! No, no, I'm not implying you should write them, god, no! It's just a way of saying how much I loved this story. :)
Anyway! This was a brilliant, brilliant crossover. FANTASTIC story.