Are you sure that Uno is using COM4?
What about your first Uno? Which COM port is it on?
With all of the switching back and forth, unplug both Unos. Power down your PC and then power up.
Plug back in the Unos one by one.
Are you sure that Uno is using COM4?
What about your first Uno? Which COM port is it on?
With all of the switching back and forth, unplug both Unos. Power down your PC and then power up.
Plug back in the Unos one by one.