The OKW Panther can hit the field quite early and it isn't OP. And considering the worse stats of the Ostheer Panther I don't really see how T4 getting a major cost reduction would really hurt much with how meh the units in the tier other than the Panther are.
I mean the Panther is a specialist tank, it's countered by AT guns and tank destroyers still have a reasonable chance to pen it's armor. I really don't feel like it hitting the field not super late in the game would impact much.
People keep saying it's a luxury, but it just feels like a chore to me.
OKW's fuel penalty ensures that that panther won't have a replacement for a long time, thats why it isn't OP. If any faction can gain the ability to build a heavy for every 1.5 mediums, it would be broken. This is why teching is a thing, to ensure that vehicles, light armor, medium armor, and heavy armor all have their respective moments of superiority. Otherwise, why not just unlock all tech buildings immediately and let everyone build what they want as thy gain the resources?
The Panther is not the only tank in T4. The Brummbar is the AI counterpart to the Panther, and it is very powerful. If you could get T4 out at the same time as you unlock T3 for example, the USF player would have no choice to rely heavily on AT platforms instead of AI. This would make Shermans lose their window of strength, and force USF players to build nothing but Jacksons to amass AT DPS. With a Jackson costing only 50 less fu than a panther, it would not be a very sustainable rate of trade, especially since USF AT guns and bazookas are less than useful in a supporting fire role against Panthers.
It absolutely is a luxury. T4 is something to unlock after you enter the late-lategame. It is there for when you are popcapped and floating fuel. It allows you to take advantage of Ostheer's superior medium armor survivability and scaling to get even better tanks out of the gate lategame.