LG C3 Inputs for Roku

So I have the following:

• 65” c3
• Samsung hw-q6cc soundbar (with eARC)
• Panasonic ub420
• Roku ultra

I currently have the soundbar plugged into input #2 (eARC) and the ub420 in #1.

It’s been a few weeks and I’m just now realizing that I can only get the Roku to play in 4k if it’s plugged into the soundbar (as opposed to input #3. This begs the question…am I missing out on picture quality not having the ub420 going through the soundbar? Or is that a specific Roku issue?