What SSID you use makes no difference at all. What’s important, if you’re trying to have as much bandwidth as possible for this use, is to ensure you’re on a channel that’s not being used by other networks. Unfortunately this is very hard with 5GHz wifi, as you’re usually limited to ch36, unless you’re in an area where you can use DFS channels and your equipment supports it.
To be clear, if you create a new SSID alongside your existing one, but they’re both using the same channel, you’re still sharing the bandwidth available, as wifi is a broadcast medium.
Also, have a read of this (Plex, 4k, transcoding, and you - aka the rules of 4k - a FAQ) it has some very good tricks and tips getting 4k streaming working as smoothly as possible.
