Will Stadia offer Age of Empires IV ? Are the any plans and in which timeframe?
Being developed by Microsoft, I really don't think it will come to any platform other than Microsoft any time soon
There is no information about a Stadia port, but as Ranma already said, the brand is owned by Microsoft and it is highly unlikely that you will see it on other platforms than PC.
So far, it only confirmed to drop in the Microsoft Store, on Steam and within the XBOX Game Pass.