I am replacing an old wall timer with a GE Smart Digital Timer. The old timer had two black wires connected to two black wires from the wall. The new timer however has red, black, white and green wires. I know the green is the ground, but how do I connect red, white and black to just two black wires?

Most likely you can't. You didn't provide a model # so I can't be certain but...

It sounds like it needs a neutral to work correctly. One that I've seen says "CFL ready." That tells me it probably has a relay inside to perform the actual switching and it needs power to do so and run the clock.

Hope it helps.

Check the instructions.

If Black is line and red is load. Connect power to black, red to load(light?), then connect the white to neutral(hopefully you have a Neutral), and green to ground.

Do you have a neutral, white wire, in the box? If you do, you're all set, otherwise...