According to RK3399 datasheet
Automatic generation of PAUSE frame control or backpressure signal to the GMAC core based on Receive FIFO-fill (threshold configurable) level
I don’t know whether this means full-duplex ethernet flow control.
Ethtool should tell you whether ethernet adapter on rock pi 4 is capable of full-duplex flow control.
What is the output of ethtool
on Rock Pi 4?