in zener regulated power supply a zener diode with vz=6v is used for regulation the load current is to be 4ma and the unregulated input is 10v what should be the value of rs?

The value of the Rs will depend on the power rating of the Zener diode to be used. If 6v/0.5w Zener is used , the maximum safe current thru Zener will be 0.5/6 = 83 mA Then minimum value of Rs will be 10–6 /0.083 = 4/0.083 =48 ohms . You may use a standard resistor value 0f 51 ohm or 56 ohm. (If you want to reduce the power dissipation in Zener diode, 120 ohms may be used which will limit the current to 33mA. (29mA thru Zener & 4mA thru Load Resistance (1.5K).