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.