I replaced my 4-pin flasher with the 3-pin some years ago. The 4-pin style is very hard to find, and not cheap when you do.
SOBill's conversion #1 (very early) indicates the tach-mounted indicator light will not work, and that is correct.
I found a work-around that gives me LH and RH tach-light functionality.
I repurposed the Orange wire found at the 4-pin flasher to function as a permanent ground for the tach light - just grounded it at the relay panel.
It had previously functioned as the ground through the 4-pin flasher operation.
So, the tach light now needed its positive LH and RH input.
I tapped into the LH and RH hazard flasher wiring under the dash, taking both to the red wire feeding the tach light.
But wait you might say, that would join the two sides together, and flash all corners instead of just one side, wouldn't it?
Not if you put a small diode in each tapped wire.
Problem solved.
Larry