How it is connected to the ignition coil? On the HV side (with any kind of sensor) or the LV (Tach) side?

If on the LV side, it's plain easy, even a PIC10F would do. Just need to detect a frequency and that's it. 4Cyl tach signal at idle is around 30Hz.

You could check on google for a Frequency To Voltage application, you find it, you're half done, add a comparator at the output of it and you have something that should work.