I was searching for the same info and didn't find many comparisons that I couldn't get from the specs of each unit. I went straight to the source and asked Spyder Robotics.
They told me that the Pro is being discontinued. The 4 has better code, has the grounded outlets and has the additional safety relay plus a few other features. I also asked about the network function on the Pro (I liked that idea) the answer was...maybe in the future that functionality may be added. I think it has the ability. (they send serial #'s to the units that way).
Spyder said hands down go for the 4.
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