Two Screws removed, rear panel does not release

I am trying to replace the battery...because my wife dropped her white iphone 4 in a glass of sweet tea.

I thought replacement would be easy, but having got the two screws out (easily), the panel did not release up two mm, (nor was I able to pry it up with a knife).

I suppose that problem is somewhat unusual, but not unique; others must have had it, too. I have been somewhat light-handed so far, in hope of not messing something up. What should I do now?