They sell a 4500 - a Twin - a Trio & a Quad...
The user guide is here...
http://www.shop.bt.com/content/uni2/documentation/498m/025455.pdf
And page 81 says...
"If you have purchased a BT Synergy 4500 multiple pack
any additional handsets come pre-registered to the base.
If you have purchased an additional handset separately
you must register it to your BT Synergy 4500 base
before it can be used."
Peccavi
June 2009