The previous answer is out dated. WiX v4 is very close to being ready and I encourage active testing of it during the RC phase.
In WiX v4, there is no installer for WiX. It’s a dotnet tool you install. If your a user of Votive, Votive is no more. Instead FireGiant has a closed source but free replacement called HeatWave. This creates your .wixproj which is now an SDK style project. References to extensions are no longer DLL references but Nuget package references.
There is a tool to covnvert v3 schema to v4 schema called WiXCop. Heatwave will do this for you in a one time project by project conversion step.
WiX is no longer installed on the build server as msbuild will automatically install the SDK package references.
FireGiant reached out to be before the release and gave me a preview look. From this I was able to update my open source project IsWiX to completely support v3 or v4 using Votive or HeatWave.
All in all it’s looking really good and once all the final bugs are gone I will be migrating all of my customers over to WiX v4.