Hello,
I have 4 wires going to the thermostat and a jumper cable between RC and RH.
At the furnace, I located the bundle going to the thermostat (the one I'm holding in my hand) but the green wire has a marrette with a black wire from a different bundle. It might be the Lifebreath HRV.
How do I connect the PEK?
Should I take out the green wire out of the marrette, connect it to the PEK, and connect the green wire out of the PEK back into the marrette with the other black wire?
This wiring should work for you.
The orange marrette in my wiring diagram is the one I marked on your picture with a red arrow. You remove that marrette and connect the black NC wire from the HRV to the PEK G wire. The green wire from the thermostat should be connected to PEK adapter G terminal. On the Ecobee that green wire should be connected to C.
What version of Ecobee do you have? Some PEK adapters are wired differently.