ZVS transformer output voltage
I don't know the switching frequency of your rig but 10uF is a quite high capacitance, the impedance of that guy at 1kHz is about 16\$\Omega\$, that is quite low.

Your simulator probably models the transformer with some inductor(s) plus some series resistor(s), these resistors resistances being in the 10\$\Omega\$ ballpark. I am guessing your switching frequency is quite higher, so the voltage you measure on the output is only a fraction of what you can have.

Use a smaller capacitor (that depends on the load you want to connect to the high voltage part) and you'll see a voltage increase up to the 4kV you were expecting.