Accidently recorded over dvd-ram

recently a friend asked me to burn him something and I forgot to take my dvd-ram backup disc out of my drive and replace it with a cdr before pressing “burn”. Now all I see on the disc is the 20mb file I burned to it but before it had over 4 gb of data. I’ve tried a few data recovery tools (isobuster, cdroller, bad copy pro) but none of them seem to be able to see anything except that last session. I know those files have to be there on the disc still but none of usual methods are working. Are all those backups just gone or is there a solution I don’t know about?

winXP SP2
550mhz 320mb ram
Mad Dog MD-16XDVD9-8X

Cdroller should do it, so they say for last version.
You don’t say if it was FAT32 or UDF formatted, but have a look to:

http://www.diskinternals.com/cd-dvd-recovery/