Will the nitrous oxide cylinder pressure gauge on my four cylinder stand tell me how much nitrous oxide gas I have left?

The pressure gauge is not an indication of the cylinder contents. Because nitrous oxide is in liquid form, the pressure gauge is only a reference that some nitrous oxide is in the cylinder. Weighing the cylinder is the only accurate means of determining the contents but this is not very feasible in practice. This is why, on a four cylinder stand, you should always have an 'In-Use' and 'Full' cylinder in place prior to the commencement of the sedation procedure.