Press M when starting.
I've found a manual ( http://fs.pc.toshiba-asia.com/drivers/public/doc/Satellite%20M640_M645%20UserGuide.exe
). It's a self-extracting exe, you'll get a pdf file after you let it run.
On page 7-3 it tells some interesting things. First, it says that there's a Windows application called TOSHIBA HWSetup, and you can manipulate Boot Priority settings from there. Note that on that screenshot it shows exactly the same four devices that you mentioned.
Then below the screenshot, there's a table, it says, press M at startup to boot from USB.
Hope this helps.