I want to select x as resistance for the protecting resistor (upper).
The zener diode can hold up to 4A
If I was guaranteed to have current through zener lower than 4A, I could consider its voltage to be 5.1V and the voltage through load to be 5.1 V.
But I'm not. So, I can't consider the voltage to be 5.1 V and I can't calculate a voltage drop for protecting resistor in terms of x, because I don't know the current in curcuit because I don't know the resistance of zener.