I am sure there are others who could give you a better answer, but it seems to me all you need to do is connect the BM5 to the BS9000 via MasterLink and then make the appropriate option setting within each.

I guess one thing to decide which might impact things a bit is whether you want to keep the BS9000 as the master or setup the BS5 to the master in the ML network.  If the BS9000 is the master, you need to press N.Music or N.Radio on your Beo4 to turn on the BS9000 when you want to play from the BS5.  The BS5 will wake up when you touch it, but it will not wake up the BS9000 so nothing will come out of the speakers until you turn on the BS9000. 

Alternatively, if the BS5 is master, it will startup when you touch it, but (IIRC), you need a fairly recent version of BS9000 software so it can work as a slave (without speakers connected to it).  Also, you may have to get an analog to digital USB converter to connect your TV to the BS5 as I recall there being some issues with listening to the AUX input on a "slave" device (that is, you may need to move the TV from the BS9000 to the BS5, and the BS5 doesn't have an analog AUX input, but you can repurpose one of it's USB ports with an analog to digital dongle).  I *think* you will be able to remap the CD and Radio Beo4 commands to N.Music and N.Radio (and then use Link-CD and Link-Radio for the CD and Radio on the BS9000).

Sorry, it's been a while so most of this is from memory and could be wrong.  Hopefully, someone will correct me.  I want to hear my CDs in my link rooms so my BS5 is a slave to my BS9000 (ML doesn't allow the distribution of "slave" audio, except for N.Music and N.Radio), but it doesn't sound like you have link rooms.

From a connection standpoint, as Razlaw says, connect them via ML, and you're done (if you're using the BS5 as a slave).  If you want it to be master, you'll move your speakers to the BM5 and possibly your TV input (and buy a dongle).