Well, it depends on the quality of the DAC I guess. Basically, that's what this is coming down to. Do you want to use the TV's DAC, the Saffire's, or the PS4's? My guess is the Saffire, but you may want to look it up, we could be surprised. If you're going to connect optical audio to spdif, I think you'll need an adapter for the different plug types at least.
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And then an optical cable for the run from your ps4 to the adapter and a coax spdif from the adapter to the interface.