The teching cost is simply too high.
Imo an alternative solution is worth trying:
Instead of reducing cost, change the units available from it.
Imo the current issue with T4 is not simply the actual high cost of teching but the combination of high tech cost, high units cost available and specialized role of units...
Having that said a I thing it worth trying leaving cost as is but moving Brumbar to t3 and Ostwind in T4. This will change the almost compulsory linear building order of Otheer allowing either to go T3 and still have access to a Soak damage/barrage support unit in the Brumbar or avoid building t3 and going for an Ostwind that can help until a Panther can be built...
It will also make Brumbar (and probably Ostwind) more attractive since currently it see little usage.