Q: Wired Connection to An AE that is extending network via wifi

I have 2 Airport Extremes. The first is a 2nd Gen model the other is a 4th Gen.

The 2nd Gen model is connected to our modem and has made a working home network for a number of wired and wireless devices.

I also have an older mac pro with no wi-fi card installed. What I'd like to do is use the 4th gen Extreme to connect to the Mac Pro via ethernet to get the mac pro on the network via the wifi Capability of the Extreme.

Using one of the other computers I have been able to configure the 4th Gen to extend the network via wifi and other wifi macs can see it on the network and it appears to be working as expected but when I connect the old mac pro I get no networking or internet through it.

The mac pro's network control panel shows it as being connected through wi-fi even though it is actually an ethernet cable into the 4th gen AE. Selecting ethernet gets me nothing.  The mac pro also doesn't seem to be getting the correct IP addresses.

Is there an incompatibility between the 2nd and 4th gen models that I'm unaware of that won't let them do this?

Just to reiterate I'm trying to go:

Modem---> 2nd Gen Extreme---> Via WiFi to ---> 4th Gen Extreme ---> Mac Pro's Ethernet.

Can it be done?

Thanks