I don't necessarily think its the tech cost that is the issue, its the cost/performance of the units in Tier 4 themselves that make them very restrictive outside of 3's and 4's and even then OKW is probably still better in that case, people play Ostheer for caches mainly.
This. T4 is affordable, but what you get out of T4 isn't good enough for what you pay.
I think the Brum will be fine after the patch (though something like +5 Range wouldn't be too bad of a thing to add).
Panther, I said this before, would be fine if it was reliable and had better moving accuracy. It's supposed to be a brawling tank destroyer, but it can't do it because it's moving accuracy makes it too big of a risk to dive due to its cost.
Werfer is garbage. The end.