This setup shows the importance of electrical isolation through the means of an isolation transformer. As long as we model earth ground as being separated by an arbitrarily large resistance in parallel with an arbitrarily small capacitance we can see that there is no feasible path of conduction between the two circuits. This means that even when we have a 4 kV potential on one side we can touch the other side with this (by using the push button) and nothing functionally happens to the lower voltage circuit (on the right).
