There is no TWAIN driver available for the FS4000US which is compatible with Windows 7.
However, you may still be able to use your scanner on Windows 7 by installing and using the VueScan scanner software: http://www.hamrick.com/
Download the trial, and if it works with your scanner you must then pay for a VueScan serial number if you want to keep using the software (or buy a new scanner instead, your choice).
Note: VueScan does not include a TWAIN driver. You can only use your scanner by opening the VueScan application, then saving the scanned document as an image file, which you can then open in your preferred graphics application.
However, I think you will find that the VueScan user-interface is far better than the scanner driver software supplied with most scanners. So good is VueScan that I bought a copy even though my scanner has it's own Windows 7 driver.