I have never seen a fat tire e-bike around here, but must say it sounds like a fantastic idea for many e-bikers. I got my wife an e-bike recently and it came with 28mm tires but I quickly realized she would be happier (and ride more :-) ) on some cushier tires so I got the biggest the frame would take, 42mm. She is scared of going fast and she has the power assist to make up the drag resistance so there doesn't seem to be any big downside to big tires for her. I don't know if I would take her all the way to 4" if I could, but 2" or 3" sure.