virgin.net (UK) 8Mbps adsl with WBG-1200A router?

i received the mail confirming a speed upgrade to 8Mbps from virgin:

Line Speed: Up to 8192 Kbps

If you have requested a broadband speed upgrade:
Your new broadband speed upgrade is now active on your phone line. For it
to take effect, you need to reboot your computer and reconnect to
Virgin.net Broadband. If you use a router, you will need to reboot your
computer and router and reconnect to Virgin.net Broadband.

i am using a Toshiba WBG-1200A router. But i consistently only get around
4MBbps speeds.
e.g using the speedtester at adslguide:

downstream 4,173.5 Kbps (=4.1 Mbps)
upstream 376.0 Kbps (=04 Mbps)

The router is 802.11b but in theory that can achieve 11Mbps. the
documentation confirms "up to 8Mbps" ADSL download speeds. what can i do?
any advice?

thx.