All Beolink1000 have the [AV] button. The mkI & II between the [PICTURE] and [SOUND] button and mkIII & mkIV next to the [STAND BY] button.
The [AV] button on the Beolink1000 and the "AV" on the Beo4 are the same.
On the Beolink1000 for example [AV] [TV] to get tv sound over the sound system (if linked and both in option 1)
On the Beo4 first press [LIST] a few times to select "AV" and then [TV]
You can however, with the Beolink1000, switch between sources by pressing [AV]
For example: both tv and radio are on and you're watching tv. The Beolink1000 is in "TV mode" and you want to control your radio without having to press [RADIO] first. You can do this by pressing [AV] [volume up] or [AV] [volume down] or [AV] [mute] or even [AV] and then followed by a number or the [/\] [\/] buttons. To switch back from the radio to the tv you do the same.
It's not possible to switch between audio and video with the "AV" command on the Beo4.