Well, there could be a couple ways. One is to run the Firestick through the HDMI on the receiver, although I'm not certain that those HDMI ports are 4k capable. The other way is if the TV has a dedicated audio out function, which I think it does. You run that to the receiver and HOPEFULLY your TV supports passthrough of the signal and you get Dolby audio.
Hopefully one of those two work for you.