I got the NanoPi R4S because i'm hoping for fibre soon and my existing openwrt modded bt hub5 isnt powerful enough for that.
A review for it is here : NanoPi R4S SBC preview with OpenWrt and Ubuntu Core - CNX Software (cnx-software.com)
And the thread for OpenWRT on the r4s is here :
NanoPi R4S rk3399 4G is a great new OpenWrt device - For Developers - OpenWrt Forum
It is Snapshot or other builds for now. It is due to be supported in the new 20 build once its released.
I'm using the anaelorlinski/OpenWrt-NanoPi-R2S-R4S-Builds: OpenWRT Builds for NanoPi R2S & R4S from official Openwrt source code with minimal set of patches (github.com) build to tinker at present.