Ugh, missed the free copy.
Not sure when the kickstarter ran, but there’s no print edition out?
Also, how compatible is it with eg. WFRP 4e material?
No, print edition hasn't shipped to backers yet.
You'd need to do some converting. Zweihander runs on a D100 engine, similar to WFRP. Zweihander has 7 Attributes to WFRP's 10 Characteristics. WFRP's Weapon Skill and Ballistic Skill are combined as a "Combat" Attribute for example. Though similar in range to WFRP, Zweihander characters' starting Attributes do start on the higher end of the WFRP scale, at 2D10+30. Zweihander Reforged has a much smaller list of Skills. Revised (the previous edition of Zweihander) had a similarly small list of "Skills," which were offset with by "Focuses," more specific applications of these Skills. These Focuses took the place of many of the Skills in WFRP (in fact, many were named for WFRP Skills). I liked this system, though I feel it was underexplained in Zweihander Revised. Sadly, Reforged has done away with Focuses, leaving a Skill list that feels too short to me. But, since Reforged is fully back compatible with Revised, I can simply add them back in.
I know there were several conversions of WFRP 2e adventures to Zweihander Revised floating around out there, but I haven't seen any for 4e. Overall Zweihander is closer ro 2e than 4e, and Zweihander Revised is closer to WFRP than the new, Reforged edition is. But Reforged is back compatible with Revised.
I'm currently running a WFRP campaign (A Private War by Tim Eccles) using Zweihander Revised. Since Zweihander has such a big Bestiary in its corebook, I've been just picking creatures from there instead of converting. Most of the WFRP stuff is there, if you squint. Human characters have required a bit more work, but not much.
The thing about Zweihander Revised was that lots of things, though renamed, were a 1:1 for WFRP. Reforged seems to move farther from that, into being more of its own thing. Which is great, but for purposes of WFRP conversions, I'd probably stick with Revised. Sure, Reforged is back compatible, but I'm a "shortest distance between two points" kind of guy (e.g., I'm lazy).