Also (a Very Un-German thing), perfect is the mortal enemy of good enough. They can make more Pz IV than keep changing for the next model every year (more Pz on the front and even more important, Spare parts, mechanics that know every trick in the book to keep Pz IV's running etc.
Again, not black and white.
The common myth, that the germans aimed for the perfect instead of the good enough... is at least misleading. They go for what they thought suit their needs - and those needs were different than f.e. the US needs.
Of course, they had their flaws and they lacked perfect hindsight, but still.
They cannot really make more PzIV troughout the war, unless they build more tank plants and do it earlier - and if you think, that should they build in the panther plants fours instead of panthers, the increase of output would have been not that great to justify the change (out of my arse: 1,2 PzIV instead of 1 panther. Hardly worth it.)
Now, of course, again, they were about 3-4 steps away to make the pzIV (and three) the best and most flexible/enduring tank in the war, but since they missed those in the design phase, they stuck with the decent (more than good enough) one they had.
Now, if you refer under the model change the subverisons, those were all good, justified changes, f.e. changing from F1 to F2 was more than necessary.
If you refer to the new model introduction again, there were immense pressure to do so. They needed something to bring the 88 to the field and after that, they needed something to keep some edge over the soviets in the pzbs after 42, needed badly.
And, maybe an often overlooked but very important factor: by 44, the gemans not only depleted they trained crews, but had to curtail their training to basically unacceptable levels. Should they not go "quality" over "quantity" the decline of german armour performance would have been more conspicuous.
