They probably feel TPUv3 is stable enough to give access to cloud customers now. But I doubt they do not already have TPUv4 versions running already internally in some capacity maybe only for testing.
Since you mention it: Cloud TPUv3 is now available in beta (brand new!) See: https://cloud.google.com/tpu/
(420 TFlop/s, 128GB HBM)
(disclaimer: while I'm part-time at Google, this is my personal impression, not an official statement, etc., etc.)