All the more reason to at least give Haxe 4 a quick try while it’s still in RC-stage, to ensure any major issues you encounter actually get fixed until the final release. 
I’m sure @RealyUniqueName would be very curious to hear about any haxe.Json-related issues, especially if they’re regressions.
Note that you can just install it to some other location and point vshaxe to it using the "haxe.executable" setting. You can seamlessly switch between different Haxe version that way.