Yes, there have been major changes done to the PyMC4 API after the development meeting at London last year. You can check everything at the github repo. It is still pre-release and the API is very much open for debate. Currently, the model follows a functional approach, with some context managers to control whether use the model for forward sampling (like pymc3.sample_prior_predictive and pymc3.sample_posterior_predictive), or for inference (like pymc3.sample).
The distributions inherit from TensorFlow Probability, but we chose to keep a more consistent API with pymc3 (i.e. distribution parameter names). Again, to get up to date with the status of PyMC4, check the github repo.