I have an ATV 3 at home and just purchased a smart lock. I was wondering if the ATV 4 does anything different for the Homekit than the 3? From what I've read, I couldn't really tell.
Nope, the only thing the aTV does is allow access if you're outside your house.
It's a completely transparent thing on both models of aTVs, doesn't even show up in a setting menu.
Currently, you can't access homekit from Siri on the aTV, although they could change that in the future.