Altima shakes at idle

Is the check engine light on?

You could try disconnecting each spark plug wire, one at a time for a few seconds each. Each time you unplug a wire, the engine should run more roughly. If any of the wires doesn't have any affect on the shaking when you unplug it, then there is something wrong with that cylinder; it's either misfiring or not firing at all. Maybe one of the injectors is dead and one cylinder is not getting fuel, or there is an intake manifold gasket leak.

If it was a dead cylinder you would probably notice the engine running rougly at more than just idle speeds, I've driven a car with 3/4 cylinders firing and at some points in the rev range it was smooth, and at others, very rough, but idle was most noticeable.

Maybe the idle is just a bit too low? About what RPM does it idle?

From what I've read it's fairly common for the #4 cylinder's intake manifold gasket to leak, causing a rough idle; you can try spraying a little throttle body cleaner around the intake runner for the #4 cylinder and see if the idle is affected at all. Apparently you can do the same test with propane from an unlit propane torch.

Hope that helps you diagnose the problem, I have a 1999 Altima with 113,000 km on it and there is very little shaking at idle, so what you're describing does not sound normal at all.