OK, if your Avant has a STB controller, you can choose your V+ box (don´t know this) if you select STB. But I assume your Avant has no STB controller or - if yes - the V+ box is not listet there.
If it has an inbuilt STB controller, you would also need a PUC cable between Avant an V+ box to control your V+ box with Beo 4, otherwise you have to control the V+ box with its own remote.
Try to assign AV (or DECODER) to p.e. DVD and connect the V+ box via scart to the AV socket (or DECODER socket). Now press DVD on your Beo 4 and you should see the picture of the V+ box. If not, connect another device (p.e. DVD player) to the Avant to see if it works.
/Räuber