I never set a 4 digit in on my tivo unit and my 2 year old has recently set the parental control. I cant remove as there is no way of knowing what he has chosen as a 4 digit pin. Ive tried resetting the device by restarting and doing a soft reset by removing the power cord with no joy. If i choose the option to clear and delete everything, it needs me to enter the pin.
The TiVo can get a code reset next time it connects to the TiVo Service.
Call up 0800 MY TIVO or email them Here to organise this, it can take a wee while to happen but it beats having to send it away. Make sure the TiVo is connected up to your home network and leave it on.
Next time, I would recommend writing your 4 digit code somewhere safe for future reference.