First, select a free HDMI input on the Beovision 7. Then, in the setup menu, assign it a source. I would select V.MEM if you want to keep the built-in digital tuner of the TV functional. Select the audio input to be also from HDMI. Then, it should work.
If you want to control the cable box with the Beo 4 remote, you will need a IR emitter cable that will be connected to the selected AV entry and the emitter part will need to be glued to the IR receiver of the box. The appropriate IR codes for the box have to be selected from the database of the TV. If the code is not included, it will have to be added by a technician using the service tool.
You can always operate the Sky Box with its own remote, however.