The 4.0 release is effectively a ground-up rewrite that was required to properly take advantage of modern web development patterns and introduce 3D capabilities. According to Esri:
It’s a big project that got underway last summer, and will likely continue into the first-half of 2017. We won't be able to supersede GVH 2.x until the 4.0 API offers all of the capabilities available with 3.16. Specifically, many of our customers take advantage of drawing, editing, capturing geometry, OGC support, and other features that are yet not available in the 4.x version.
For this reason, we will continue to release additional versions of GVH 2.x before GVH 3.0 is released, as there is lots of useful work for us to do that'll see heavy use by customers in the full-featured, 2D-only space.
The day will come when GVH 3.0 supersedes 2.x; at that time, we’ll enable our customers to simply upgrade their applications from 2.x to 3.x, and start leveraging all of the great capabilities that the 4.x ArcGIS API has to offer... without losing any functionality or having to rewrite applications (except for custom code they've created).
Applications – or aspects of applications that leverage configured modules or provide custom functionality via Geocortex Workflows -- will be directly upgradeable. Customers and partners that have written custom code on top of GVH 2.x to solve unique and specific use cases will likely need to update or rewrite that custom code to migrate because 4.0 is a fundamentally new API (this scenario highlights why our enduring product strategy and development recommendation is to follow approaches that avoid custom code as much as possible).
We work hard to build technology that helps ensure you can get maximum work done today, while also engineering our products to ensure minimum possible challenges when it comes time to embrace and leverage what's next. We believe our plans for 4.0 are right on track.
We'll share more in-depth information about what's coming with Geocortex Viewer for HTML5 3.0 (and much, much more) during our Geocortex User Conference in a couple weeks (May 18-19), so don't forget to register!