1st thing I'd suggest is a direct connection to the modem.
Then go to
http://gamefiles.blueyonder.co.uk
Start 2 downloads of big files, sit them side by side and add the transfer rate of each together.
This is the best method of testing your download speed.
Then try again with router connected.
Apparently the SB4200 can give 20meg, but if you can't get this speed during the night, I'd be looking at requesting a new modem.