You absolutely must consult your vehicle's specific repair manual. This manual will provide the correct torque specifications for your particular engine and year. The manual is crucial because using the wrong torque can damage the rocker arms, valves, or even the engine itself.
You can usually find a repair manual online (often as a PDF) by searching for "[Your Year] Ford Ranger 4.0L Repair Manual." Alternatively, you can obtain one from a local auto parts store.