You do not need Apple TV4 to use HomeKit on your own LAN. You only need Apple TV4 if you want to control HomeKit remotely away from your home. For example; say you have a Ecobee 3 thermostat (HomeKit version) and you want to control it via HomeKit while you are home. It will work without a Apple TV4 via a iPad and/or a iPhone. However, if you are at work and you want to control it, then, you will need a Apple TV4. The current version of Apple TV4 should be able to do this out of the box.
I have homekit enabled products with a 3rd gen apple tv and i can control everything when im not at home. Homekit works with both the 3rd and 4th gen apple tvs