Hi Bill
Roastmaster addresses Temp Sensors by the serial numbers of the temp sensors themselves (not the SBC) - so you can use multiple Temp Sensors on the same SBC.
So, first you'd tether the 4-port with the new probe to the SBC via the USB cable, and create the second probe definition in Roastmaster with the new 4-port serial number/port.
The, you could either leave the 1-port with its probe in place (having 2 temps sensors on the SBC - each with its unique serial number), or disconnect it and hook the old probe to the 4-port, and update the old probe definition with the new serial number.
I'd probably opt for both probes on the 4 Port - it's a little less processing power/bandwidth, but either way would work.
Danny