Here are the reasons:
There are quality issues with our current integration that cannot be addressed in the context of 3.x, because Signalyst does not plan to back-port the bits required to fix them to the 3.x API.
There are deep enough changes involved in moving to v4 API and addressing these issues that it is impractical for us to support both side-by-side. This would also put us in a position of supporting configurations that we know are broken, which is the whole thing we are trying to fix by doing work on our HQPlayer support.
We are not going to be supporting earlier versions of HQPlayer 4.x for the same reason–they have known issues that make the experience of using the integration less smooth, and we cannot be in a position of supporting broken integrations.