Stopping point Vienna to Prague

I guess it would depend on what type of city/town you want to visit for a few hours. Cesky Krumlov (small town, popular tourist stop) is not on the fastest most direct route between them but it would be a fun place to stop for lunch and a couple of hours exploration. Brno (large historic city with much to see) is on a faster, more direct driving route. I loved CK but I spent the night so was able to enjoy it without massive tourist crowds. Even with that, I would recommend it. Have not been to Brno but have heard good things about it from people here on the forum familiar with it.

I agree with Nancy. C.K. is much better if you can stay overnight. Brno is pleasantly non touristy lively city. It did not make it into Rick Steves book yet. With 4 hours you can at least walk the main shopping street called Masaryk Street from the Main Train Station to namesti Svobody (Freedom Square) and then continue on Ceska Street. On the way back visit St.Peter Cathedral and go up the tower for views. If you still have time to see Spilberk Castle which is close to the downtown, even better.

Visit the Moravian Karst cave system, just outside Brno. Access to the cave system is by guided tour, which is half on foot, half on by boat on the underground river system. It is one of the most spectacular places in the Czech Republic. Not to be missed!

Thank you for the tips. I had CK on the itinerary but after doing several castles through Germany and Austria I was looking for maybe something different. I will still keep it in mind but also look into Bruno and the caves.
Thanks
Rick C.