1. My BeoVision 8 has two ir-control sockets, identified in the menu as SC1 and SC2. These provide translated signals for a variety of peripheral units, and let you control them.
2. You'll need the ir-eye connector kit (just a cable with a minijack at one end, and an ir-eye at the other. You place the ir-eye across the ir-eye on the AppleTV. (Don't cover it entirely, as you may want to be able to use the Apple remote for some reason. Leave a little showing underneath the B&O ir-eye you're affixing to the AppleTV.
3. Make sure your BeoVision has the software installed that lets it control the "ipodremote" - this will let you send the appropriate signals to the AppleTV (or another Mac device that understand ipodremote.) Your dealer can configure this if it's missing. In the BeoVision's menu go to Connections, and then to the AV port you've connected the AppleTV to - there you'll find IR SOCKET. Pick the socket you have connected the ir-eye minijack to, and pick ipodremote after you press GO with your Beo4.
When this is stored, you should be able to control the AppleTV with your Beo4.