I took a test ride on both last season over at the local zombie Trek shop. I already had my gsd for comparison and was just curious. The fetch 4 felt gigantic and overbalanced to me. Especially now that I've ridden a UA for comparison a couple of times. The fetch 2 on the other hand felt like a really smart, powerful bike, much less overdone. I haven't tried a lot of Trek ebikes, I don't love the look, but if you're a brand loyalist looking for your GSD killer, it's a good bike. They went premium with Bosch, a design choice I prefer over the proprietary in house motor approach taken with the Globe Haul (which is by all accounts still a fantastic ride).
I wanted Trek's cargobikes to be successful, so they'll do more of them. Maybe they need to fill in with a Fetch 1 and 3...