Be careful when increasing the delay - especially the end of transmission delay - as you can end up in a situation where your RS485 transceiver is still in TX mode when the remote device is ready to respond.
I've not got an R4 but I wonder if there is an issue with the way the serial port is implemented such that the flush() command isn't working as it should.
I've just found this discussion:
which may be the reason that flush() isn't working correctly.