Engine idle speed

Not sure what you mean here. Is the engine idling at too low of a speed or do you wish to adjust the idle so it idles at a lower speed because it's running too fast? If it's the latter, the engine control computer has complete control over the base idle speed and it is not adjustable at all. However, if it IS idling too fast then this means that the engine control computer has reached the limit of what it can do and the still too high of idle speed is being caused by a mechanical cause, usually a vacuum leak. I don't believe I've come across a 4200 that idles too slow but I've seen multiple vacuum hose leaks that have caused too high of idle speeds. Hope this helps.

most older cars had 2 adjustments , a air screw for enrichment and throttle screw for engine idle speed, later models did away with the enrichment screw, to adjust set idle screw to 600 rpm, adjust air mixture screw to fastest rpm , reset idle screw to preferred idle speed.

You can't adjust the idle , There is a minimum idle adjustment set at the factory , you don't want to mess with that . The PCM / ECM ,engine computer controls the idle speed by the idle air control valve (motor) . You may have a dirty throttle plate mess with the base idle .Check out this video on ford base idle adjustment . Some one who didn't know cranked up the base idle causing problems Ford IAC valve test how to adjust minimum idle speed

Its not really adjustable.
The computer controls idle speed via the idle air control valve.
You probably have a vac leak.

you can adjust the idle air right at the air intake

You probably need to adjust idle speed

An IAC (idle air control) motor is designed to adjust the engine idle RPM speed by opening and closing an air bypass passage inside the throttle body. The cars computer or ECM (electronic control module) receives information from various sensors and will output signals to adjust the IAC motor in or out to adjust engine idle speed by controlling engine idle air. ----------------------------------------------------------------------------------------
XPROG-M V5.0

3-13 Degrees
Application:
With manual transmission

Note :
Before top dead center, at specified curb idle speed.
Locate data link connector & connect jumper wire
between ground & engine test terminals.
After completing adjustment, remove jumper wire.
Curb idle speed (750 RPM) transmission in drive.

3-13 Degrees
Application:
With automatic transmission

Note :
Before top dead center, at specified curb idle speed.
Locate data link connector & connector jumper
wire between ground & engine test terminals.
After completing adjustment, remove jumper wire.
When adjusting idle speed, set parking
brake & chock drive wheels.
Curb idle speed (750 RPM) transmission in neutral

Idle Speed Control (ISC) Motor OPERATION Eagle Throttle Body Fuel Injection (TBI) System NOTE: The Eagle TBI system is used only on 2.5L Premier engines. The Idle Speed Control (ISC) motor is mounted to the throttle body and controlled by the PCM. The throttle lever rests against the initial adjustment screw of the actuator (plunger). The idle speed is not adjustable. The initial adjustment screw is only used to establish the initial positioning of the actuator when the ISC motor has been replaced. The actuator extends or retracts to control engine idle speed and to set throttle stop angle during deceleration. Based on inputs from the various engine control system sensors and switches to the PCM supplies current to the ISC motor to adjust the actuator position for the particular operating conditions.
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The engine doesn't have a mechanical idle speed adjustment - VW/Audi have a software called VagCom (the diagnostic software they use) that is used to adjust the idle - it's completely computer-controlled. You have to change the idle speed using the software.