We're about to start development on the new Domain Manager system. To be perfectly honest, v4 took way longer than we expected. It was much more involved than we thought it would be, and it pushed off domains.
We're taking a hard look right now at all of the discussions about domains, and it looks like we're going to stick with the KISS principle. (Keep It Simple Stupid). Even so, improving domain support is a big deal and we're not going to let it prevent us from coming out with other releases while we work on it. It's likely that the domain system will be implemented almost entirely as a plugin, though there are some core changes that are necessary like packages being able to defer their pricing to plugins. Aside from any core dependencies, the domain plugin may be able to be released as a beta on its own where we look forward to getting more feedback from the community.
Since there are also other important things that need to be done as well, we are going to work on those things simultaneously and reduce the time to release. Domains will be included when it is ready, but we aren't going to hold back releases as a result.
I hope that makes sense, and I understand how frustrating it is not to already have better support for domains. Just know that we're working on it, alongside other things, and will release it to the community when we have something ready to test.