I thought you are driving primary as flyback converter. Vin*D/(1-D)=Vout.
With 10:160 turns ratio and 50% duty, you can get output 16 times of input only.
The reason why you get 2.5KVDC because you get the HV by leakage inductance of transformer, only a very small load at output will cause voltage drop quickly.
When you connect the 4 circuit in series, the output HV will discharge air causing energy loss which act as a loading. when you use instrument measuring the HV also add loading to output, that why output can’t get the exact voltage 4* 2.5KV.