I'm not sure about 4" becoming the norm, but the ebike front wheel kit I got (350 watt Hilltopper) came with wide'ish rim, that required a wide'ish (40mm?) tire. My guess is that a wider tire was required so that it had a bigger contact patch for traction, especially with a throttle-only option. And I'm guessing that an ebike with more torque (than my 350w), say 500w, 750w, or 1000w will require an even WIDER tire.