A level physics Electricity question (2)

Voltmeters measure the potential difference across the ACTUAL voltmeter, not the Resistor in parallel.
Usually they are the same thing but not if the resistor section is disconnected.
So in the open switch case you can think of the voltmeter being in series with the battery, so there are 4.2V across it.

When the switch is open, we can ignore the path through the 10 ohm resistor. I recommend drawing a circuit without the 10 ohm resistor and that shows that the battery, voltmeter and the "system" of resistors is in series. Another way to look at it is that the voltmeter in this case measures the voltage across the system of resistors. The two resistors have same voltage and considering our circuit is ideal, the voltage across the system will be 4.2.