Need to isolate panel meters from each other?
The 9V supply is only used to power the meter itself. If your 9V supply can power all the four meters, that's OK. From the specification of the panel meter, it need 20mA to work, then all your four meter will need 80mA or so. Your DC-DC isolated converter can output 111mA max. So it should be OK. 



Update:

The meter may measure voltage higher than it's own supply voltage, so it recommends you to isolate its' own supply from the circuit will be measured. So the key point now is you need a safe 9V power supply isolated from the circuit with higher voltage. Once you get such a supply, many meters can share it, given your supply can source enough current. Now, every meter is isolated from the high voltage, so it's safe, and safe to each other. You needn't isolate their power supply from each other.