Frank,
On many scanners there are screws under the lid hinges which hold the
top on. The lid often lifts off allowing removal of the screws and the
top of the unit. I don't know the 4870 but as no one has answered I
thought this may be a little help.
Incidentally if you do get in when you polish off the glass do the
underside with strokes at 90° to the top then, if you have left
streaks, you can easily tell which side they are on.
Michael
