Old Home Theater with new 4k TV questions

Quck history...I purchased a simple Harman Kardon home theatre 5.1 setup back around 2011 I believe. Due to the nature of my job and moving around a lot it's basically been sitting in the box for the past couple of years. Christmas this year I purchased a 4k tv, and lately i've been wanting to setup the 5.1.

So, my question is since the HK receiver I have is pre-4k tech and I can no longer use it as the switch (plugging all of my devices into it, cable/ps4/media streaming device etc) I'm wondering what the simplest work around would be so that i can still take advantage of the surround sound?

I've done a little research, and if i'm understanding it correctly, i have plugged all of my devices into the tv's 4k compatible hdcp 2.2 slots, and all i need to do is run an optical cable from the tv into the optical port of the Harmon Kardon receiver to utilize the surround sound setup? does this make sense? so i'll just bypass the tv's speakers and use the home theatre's speakers. also, can i just use the ARC input/output on the tv & receiver to do the same thing as decribed above? if so, which would be the better option, hdmi ARC or optical cable?

This seems like a pretty convoluted question, so I hope someone can make sense of it.