Has anyone tried connecting to Mach3/4 via modbus using wifi?  Technically, it should work just like wired connection.  Just wondering if there is something about it that would required a wired connection.

Usng WiFi for a any quasi-real-time device is a bad idea, especially if the WiFi router is serving more than a couple of devices. The connection negotiation protocols can generate unpredictable and sometimes significant delays.

All of that considered though, is it possible?  Or does Ethernet modbus require a wired connection?

Technically it should work if you get the IP addresses correct. You may have to do
this by configuring the router to "open a port" with a fixed address.