The tach is essentially a pulse counter that provides a given angular
deflection of the pointer for a particular number of pulses per unit time.
The tach will function just fine using six pulses per engine revolution but
of course the pointer will be off relative to the markings on the face.

At 1000 rpm you would have 6000 ppm.  The tach will deflect to its 6000 ppm
angle which for a four pulses per revolution environment would indicate 1500
rpm.  At an expected 4000 rpm you would see 6000 indicated on the face of
the tach (combining the pointer deflection and the face graphic.)

Several alternatives come to mind.  If you have an SVX motor the ignition
system allows selecting a subset of the pulses to feed the tach.  Pick four
of the six and you should have an "accurate" reading.  But since the face
graphic will be in appropriate for a motor that comfortably revs to 6500
changing that face graphic may be just what is called for after reducing the
number of counted pulses.  Smallcar.com has graphics for sale.

Or...visit your local purveyor of experienced volkswagen parts and get a VR6
or AUDI V6 tach.  Try it and see if it does what you have in mind.

In my particular case, I use a tach from a 5cyl and count five pulses per
revolution using a diode summing arrangement.  Works fine.

Best alternative is to purchase an aftermarket VDO or other tach designed
for multiple applications that is suitable for a six cylinder environment.

cheers,
pensioner (svx with audi tach)