So, I went for the Wunderlich in the end. As Alex noted popper fittings all around - even at the back under the handlebars, which should be more convenient than fighting the velcro strap on the Gen 3 bag. Here are some pics of them together.
The Gen 4 bag is narrower and is taller - not as taller as marketing pics suggested, it just fits the profile of the tank more, so has a more pronounced curve to the underside. It looks like it'll fit the same amount of stuff, which is good. When you open the zip it opens the whole thing which will have it's pros and cons vs the the old bag, where I liked the sides being slightly more difficult to access.
I have never suffered from tank scratch as others have reported from Gen 3 bag (which I have used since 2017), but started using some rubber webbing under the bag when in use, so it'll never bother me.