How many pictures on 4GB memory card in 12.3 mega pixel digital camera?

It depends entirely on the camera setting for the pictures you're taking. You can set the camera to take hi-resolution pictures, in which case you can take fewer pictures, or lower resolutions, letting you take more pictures. Generally you want higher resolution as you can edit them and there's no compression artifacts (squiggly lines/blurring).

Check your camera's user manual, then look for the section that deals with file sizes. It'll tell you how to change the file size so you can get more or less from your card.