At the last event "PTC LiveWorx Europe 2015" in November of 2015 it was presented a roadmap for the development of Mathcad Prime 4.0 and some future steps (themes) for Prime direction. PDF "PTC Mathcad Roadmap_2015" can be found in attachment. You can see what updates are planned for version 4.0 (now the release of version 4.0 is scheduled for the 4th quarter of 2016 year) and is scheduled for subsequent releases after 4.0.
Update_June 2016: New timeline (at time of June 2016) for Mathcad/Mathcad Prime product:
Update_Jule 2016: In June of 2016 year within an annual meeting conference "PTC LiveWorx 2016" the new program of development for Mathcad Prime was submitted. For future release of Mathcad Prime 4.0 the same new planned features were shown in the program about which I wrote earlier here (except for expanded options of creation of 2D plots - this feature is transferred it for 5.0 version): Mathcad Prime 4.0: some new updates
PDF "PTC Mathcad Roadmap_2016" can be found in attachment.
Update_February 2017: Added the presentation about Mathcad Prime 4.0 from the PTC Forum Europe which was in November 2016.