I would put more emphasis on the playback machine rather than the router - the RT-AC68U is still a very relevant device, and replacing it without looking at the client side is a bit short-sighted. Just replacing the router, you may not see any difference at all.
Questions to think about:
- What are you doing 4K playback on and what kind of media?
- Are you doing HEVC and/or H.265?
- What bit rates is your content coded at?
I've got a little AllWinner H3 based Android Box (Beelink X2), that nominally will do 4K playback (just barely), but definitely struggles with it's built-in WiFi (it's 802.11n 2.4GHz)...
Replacing the router would not make things any better, as it's client limited.
The router just carries bits - and the RT-AC68U, like any AC1900 class Router/AP, does a fair job of that - with 4K media, it's more about the display device and content.