Leccy rev counter

Here's a question for leccy experts.

I bought an electronic rev counter off eBay, the guy said it works on 2 and 4 stroke engines, now, I'm a bit slow when it comes to wires etc but! the rev counter runs off a coil, so if a 2 stroke coil pulses every 2 rev and a 4 stroke coil pulses ever 4 revs how can the rev counter work on both engine and read the revs correctly?

Surely there must be two types, 1 with a 1:2 ratio and 1 with a 1:4 ratio.