Ground loop perhaps.  Is it possible to get all of the speakers and the iPod dock on the same electrical circuit?  Second possibility: Light dimmers on the same circuit as any of the other pieces involved.

I had an Ipod connected to my BL 8000, the plug for the Ipod was very hard to connect, not properly inserted it gave "hum"

Note I'm yet to install the wiring and speakers...

To test before I install and wire up, I used an ipod to powerlink cable in to one speaker. I then used a powerlink cable to link each speaker. I set the switches on both to Line and the speakers hummed.

So before I install wiring and mount the speakers I need to ascertain how to wire them so they don't hum....

Which BL4000 version are they?

the ones I've tested are Mark 2, but I'll be installing a mix :/

When I tried my Ipod direct to BL 8000s, i had a cable that connected to the Ipod, then from the ipod with one cable to each speaker to the line input, as far as I know you cannot use powerlink cables with the speakers set to line. With the powerlink cables there is a trigger current triggering the speakers, the Ipod does not have that function.

If you want to drive 3 sets of speakers, I suggest a simple speaker distributor with one left right line in, and 3 left right lines out, from there to line in on the speakers. and speakers set to line. I have only tried with one set of speakers, but what the Ipod gave out the speakers played, and with good volume too. Howe it will be with 3 sets I really dont know. Maybe, Ipod to an amplifier, distributor, and then speakers, would work better.

Jonathan wrote:

Whenever I try to use the Beolab 4000s directly I get a humming sound

PowerLink cables (like "Line" cables using RCA/Phono plugs) are unbalanced cables.

On longer distances these cables, especially the "thin" versions will pick hum.

Double shielded cables (like the recent 5mm B&O PowerLink-cables) are more resistant to hum.   

Double grounding causing ground loops can also produce hum ...

BeoLab MK1 and MK2 use different power-supplys, a mixture of both types on the same output can also
lead to hum ...

A solution could be an old BeoLink converter as "Audio-Master" and 3 BeoLink Active for three zones
wired via ML Cable. MasterLink is a symetrical = balanced cable (same technology beeing used in
radio stations, theatres, recording studios and PA) and hence quite resistant to hum.

Only the remaing lenght from BL-Active to the BLb 4000 will then be an unbalanced cable.