A timer won't help much, as the time between sparks depends on how fast the engine is going.
How you deal with this also depends on how the ignition circuit is arranged. I've seen 4 cylinder engines with 1, 2 and 4 coils. If there is one coil, you need to stop every 4th spark. If there are two coils, you have to stop every second spark. If there are 4 coils, just disconnect one of them.
Is it vital to make sure that a particular cylinder fails to fire? If so, you can't just skip every 2nd or 4th spark, as every now and then you will miss a count and it will be a different cylinder that is missed. Therefore you'll need some way to keep it synchronised to one particular cylinder if that is important.
A lot of cars have the ignition drivers within the ECU, so you may not be able to find the connections easily. The ignition coils store the energy needed for the spark, and the spark happens when the current is turned off, so you have to turn off the signal for a variable amount of time, depending on engine speed.
On most modern cars, you can easily burn out the catalytic converter by running with not all the cylinders firing, as the unburnt fuel will burn in the exhaust system. The ECU may go into a reduced power mode and register a fault if one cylinder isn't firing.
It may be easier to disconnect an injector, as most modern petrol cars have one per cylinder.