Pzkpfw IV Ausf G-J is Germany's primary medium combat tank. It's full name is Panzerkampfwagen (Armoured Fighting Vehicle) IV, with Ausf denoting Ausführung, or variant. In-game, the Panzer IV represents variants G, H, I, and J; these variants were all very similar, at least in ways that could be represented in the game, and so are lumped together.
The Panzer IV began life as a directive established in 1934 for a heavy support tank, and is considered a "brainchild" of Heinz Guderian, a leading German tank tactician and commander in the years leading up to and during the war. Originally designed as an infantry support tank armed with a low-velocity, short-barreled 75mm gun, it was modified to carry the long-barreled 75mm KwK 40 after the invasion of the Soviet Union to counter the Soviet T-34-76. From 1936 to the end of the war it is estimated roughly 8,000-9,000 Panzer IV's were produced.
Compared to other medium combat tanks such as the M4A4 Sherman, Cromwell Mk IV and T-34-76, the Panzer IV has far superior offensive capability, with a longer-ranged, more accurate gun with excellent penetration capabilities. Defensively and maneuverability-wise, however, it is somewhat inferior to its contemporaries and is outclassed by the Allied advanced medium tanks (Sherman 76mm, Sherman Firefly, and T-34-85). Like most German combat vehicles, the Panzer IV is best suited to longer-range combat, keeping enemy tanks at a safe distance. However, once one has acquired better tanks (Tigers and/or Panthers) the Panzer IV's smaller size and lower cost make it a more attractive unit to assign to risky close-combat maneuvers or sneak flanking attacks through confined terrain.
Unless fighting a very mobile enemy, stugs are almost always a better choice, as they are significantly cheaper and have great front armor, all while fireing the same weapon (the starting Anti-tank gun has the same weapons as well, to give you an idea of why this is good). Panzer IV's are, despite being the main combat tank for the mid game german, are rarely made in large numbers, as they have a specialized Stug for defencive lines and Tiger I for heavy weaponry. Panzer IV's take the middle ground: when you need both, but can't really afford it.