Clearly June 1944 was an inflection point in German armored strength. Despite Allied bombing and reversals in Italy and on the Eastern Front through 1943 and early 1944, the German Panzer force continued to grow. However, June 1944 saw both the Allied invasion of Normandy and perhaps more importantly from a pure numbers standpoint, Operation Bagration in the East. The chart of Panzer loses below shows a sharp spike in June-July 1944 coinciding with these operations.
From a pure numbers standpoint it is clear that at the time of the Normandy invasions and Operation Bagration, the Panther was just as common as the Panzer IV on the battlefield. Past that point, the Panther actually became more slightly more common than the Panzer IV. Another point to consider is the fact that armored divisions typically were organized using a single model of tank at the company level, so Panthers and Panzer IV's would each form their own companies, so "Panther heavy" German armored companies do actually have a strong historical basis.