Many older (and/or damanged) PzKpfw IV tank were modified into close support guns for the infantry.
Their inferior armor was enhanced by adding plates to the sides.
As with the Jagdpanzer IV the turretless design allowed for bigger guns to be mounted on the chassis.
The Jagdpanzer with its moderne design using sloped armor was perhaps a better fighting vehicle, but the StuG IV was simpler to build and readily available.
This version shows a modified position of the exhaust pipes.