How much 14.1MP video can a 4GB memory card hold?

It depends on the detail in the picture ! The more detail a photograph has, the more memory is used to store it.

Think like this - A picture of a blue car against the brown brickwork of a house wouldn't use much memory, as the car & house are large 'blocks' of uniform colours. Put the car in front of a tree in autumn, and there are hundreds of leaves, all different colours - needing more memory to capture the image.

A good average at 14 mega-pixel resolution would be around 160 images.