I use this combination in Mach4 successfully, ESS runs the machine, the PoKeys does my physical buttons and pots (SRO/FRO). I, too, replaced a PoKeys 55 that was on Mach3 when I upgraded to Mach4. I have the PoKeys57e simply connected to an Ethernet switch on my home network, it doesn't require a dedicated NIC like the ESS prefers. I have two Ethernet ports in my PC, one for the ESS and one for the network/internet (GASP!) This combo has been running flawlessly on Mach 4 for a couple years since I sucked it up and switched to Mach4. If you're switching from Mach3 to Mach4, get ready for your machine running much better, but also be prepared for a bit of learning/unlearning!! Mach4 isn't an upgrade to Mach3, it's a total different thing.
If you've written code in Mach3 for your buttons, you'll have to learn a bit of LUA to do it in Mach4, but that said, the PoKeys plugin has many of the buttons you may be using in a tab in the plugin. I haven't tried those because I wrote the code before that tab was in the PoKeys plugin and I haven't taken the time to go back and redo it.
Good Luck, it'll be worth any trouble and DO get the Ethernet PoKeys!
Tom