dplex

That would work too, but nobody just has opto couplers lying around

They do

http://www.digikey.co.nz/product-detail/en/SFH618A-2/SFH618A-2-ND/1731586

With a clean and stable 4.1V input signal you could simply exchange the input filter path against an optocoupler (see below for example).

With a Megasquirt setup, the tach circuit should work without modification using the units tach output though

Good luck,

Adrian