@classicalgeekAre you really sure that this card is
NOT a SDHC one ?
The A540 doesn't support SDHC cards, only SD cards are usable.
There are both types of memory cards with 4GB out there, the older SD type and the newer SDHC type.
Normally on a SDHC card there's a classification on the label, e.g. "Class 6" or just "(6)"...
When you start such a camera with a SDHC card, you'll always get "Memory Card Error", also "Card locked" which dissapears after some second (in playback mode).
But then camera should
never accept this card !?
So when the camera itself don't accept the card, either it is a SDHC one, or the card is broken...