Can't build a level 4 tower

Maybe I’m missing something, we have the -15% tower costs tech, level 10 stone  (storing 49.7k stone), but a level 4 tower costs 51k, so, even if we wanted to, we can’t build it.

Is this on purpose, or a miscalculation somewhere? (cost is way too high anyway=)).

I know there is very little economy to increasing tower range.   It is like a cost per tile regardless of the tower level.   Right now there is too much risk and too little reward to larger towers.    Players will attack the bigger tower for the gain of taking it down but the cost of 3 little towers is about the same.