I didn't make QoS changes to the default dd-wrt settings (I believe QoS is off by default).
Granted, I have a different ("main") router which provides wifi for some of our devices, so not all the load is on the secondary router, which connects the nas to the "streaming clients" (we don't have the need for more than one at a time right now).
But overall, I would expect any decent router to be able to stream 4k, of course assuming that there are no wifi congestion issues and that the material bitrate is "reasonable" - 4k uhd disk material tops out at 128Mbit/s.
I would expect that there might be issues with throughput consistency - my shieldtv has wifi issues, but I have configured Kodi with 400MB buffer and that helps, although I don't have that option on my oppo player - but the oppo doesn't need it either.