How many pictures on a 4gb memory card in 12 megapixel camera?

Well this really depends on your camera and the image size that you are going be shooting and whether you are going to shoot in .jpeg or RAW but
if you are talking about regular digital cameras on a medium size image, you can take about 600-1000photos
but on something more serious like a DSLR, in case with my Nikon D300s, you can only take about 200shots RAW.