Would you please recommend me a tachometer for idle speed adjustment which can go down to 420 RPM?

Thanks!

1, 3, 4, 6, 8, 12 cylinder? Spark plug pickup? You might have to do a little math. Like you can use the 8 cylinder setting for a 4 cylinder. Just multiply by 2.

4 cylinder with a spark plug pickup would be nice. Any recommendations?

I found this one with a little searching:

http://www.stevensinstrument.com/tachometers.htm

Non-contact. Remember that some cars fire the spark plug on the power and exhaust stroke when they use 2 coild for 4 cylinders.