PyMC4 on TFP was the plan, but they made a recent announcement [1] indicating those efforts would stop, and instead, they would develop PyMC3+JAX+Theano.
Woah. Thanks for the link, as a PyMC3 user I was not looking forward to the transition to 4 expecting to have to relearn the API like the transition from 2 to 3. I was debating wether I should learn 4 or switch to a different library when all I really wanted to do was stick with 3.
Looks like I get the best of both worlds now.