How do I adjust 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.

The idle speed is controlled by the engine computer through the idle air control valve , there are no provisions for adjusting idle . If idle is to high or you just think it's high , you need to test ,don't guess . Watch this video that shows what happens when you adjust thing's that you should not . Ford IAC valve test how to adjust minimum idle speed How to do it correctly !

It seems like idle is too low. There should be a screw to turn and adjust idle speed.

The idle speed is controlled by the computer.
Did you check the timing belt to see if cam timing is correct ?

Idle speed is electronically controlled and not adjustable, So if the engine seems slow you have a problem. I suggest you do the following service to improve this concern.

This problem is commonly caused by a dirty automatic idle speed control valve and throttle valve. Buy a can of throttle valve cleaner (do not use carburetor spray cleaner!) from NAPA or Carquest (made by CRC chemicals) and spray it into the air intake while the engine is running, use up about 1/2 the can, engine will try to stall hold the speed up, shut it down and let it soak for 30 minutes, restart and blow out the remaining fluid, shut it down and disconnect the negative battery cable for 5 Min's to reset the base idle control

You can adjust the idle screw that is at the end of the accelerator cable to increase the idel speed.

You do not need to reset the computer. The idle speed is low because the computer was reset when the battery was disconnected when the T belt was replaced, the system will slowly adjust the idle back up to the correct speed based on your driving habits, it learns as you drive. It will take about 100 miles.

PROMLEM MAY BE BECAUSE LOW IDLE SPEED ADJUSTMENT. INCREASE THE LOW SPEED/ IDLE SPEED BY INCREASING THE FUEL INTAKE . IN CARBURATOR VEH. ADJUST THE IDLE SCREW CLOCKWISE. IF IT IS FUEL INJECTOR INCREASE THE FUEL AT PUMP ITSELF.

Idle is factory set and cannot be adjusted, your best bet is checking for cracked hose pipework for air intake at idle,(not at problem under exceleration and driving) Any slight upset to the air /fuel ratio will be most prevelent at idle speeds.Check hoses where the meet metal flanges on manifold intakes and any sharp bends in ppipes.Good luck

heve utried adjusting the trottle , just put your foot on the accelerater and hold at just under 1000 revs if shaking goes then thats your problem should idle at just under 1000rpm