I don't own an R4 Wifi, but do you need software serial? From what I've read in a post by @UKHeliBob quoted below, it seems that if you are using pins 0 & 1 for your RS485 comms then maybe it’s not required :
In addition, you can then use the flush() function to wait for the transmission to complete before switching back to receive.
As you are also using external hardware, you should provide details of how you have connected that up to you UNO R4.