If the serial port is available, add a Digi One IAP to the system. It will allow the 5/04 to be accessed over an Ethernet connection. It is similar to the Allen-Bradley 1761-NET-ENI. The Digi unit is more versatile and has performed much better for me as well as being half the physical size.
We recently bought a DataLink GW1000-ABEIP for an Ignition project done by a subcontractor and as far as I know, it is working well. In our case there are only three nodes on the DH+ network, a Panelview Standard, a 5/04 and the GW1000. I was not involved in the nuts and bolts of setting it up.