The tachometers count prop blades... they don't measure tip speed, so blade length is irrelevant.

The tah will have a setting for 2-blade and for 3-blade. Very few people make use of the 3 blade function. And the (even fewer) people who use 4-blade props simply divide the indicated rpm in half.

There is no difference in how you use the tach on the 4-stroke vs the 2-stroke. Turn it on, point it at the engine through the prop arc... or point it through the prop arc from behind... It does help to have a good light source on the poosite side of the prop from the tach, though they can usually read the reflection of the light off the blades, at any angle when outdoors on a sunny day...

The rpm will vary with each individual engine, and with each individual prop on a given engine. The tach is needed because its harder to hear when a 4-stoke is running at peak rpm, than it is with a 2-stroke. Find the peak rpm, and richen the fuel mix about 1/4 turn durring break-in.

You should see the exhaust as wispy white trails behind the plane in flight. If you can't see any exhaust trail against a clear blue sky, SHUT DOWN AND LAND. 4-strokes will run WAY lean, and burn themselves up. 2-strokes will die from fuel starvation long before they will get as hot as a 4-stroke will get durring a lean run.