hi 10,
If you decide that you want to build the device, let me know and I will post a more detailed/optimised circuit.
The circuit posted above gives a 2:3 ratio ie: 4 pulses in with 6 pulses out.
The method is the 4 pulses are doubled to produce 8 pulses, then the divider blocks 2 of these pulses, so the output is 6 pulses.
But I must know from you the input signal specification. ie: voltage levels from the 4 Tacho source
Also the output drive voltage/current required to drive the 6 Tacho.
E