Thanks Mohamed - OK, so the design consists of an oscillator, transformer and a Cockroft-Walton voltage multiplier chain.
With this type of approach you'll be able to deliver an attention-getting (transiently painful) *SNAP* to anyone who touches the output terminal(s - don't forget to provide a ground contact near the HV output), but it won't enter into the "stun" category unless you seriously raise its output capability. Typically this is done via a high voltage pulse transformer output stage instead of a capacitor bristling with voltage
Nonetheless, there are a few things you can do to make it a little more "bitey"...
* The transformer's output capability will be [potentially] limited by the inadequate current sinking ability of Q1 (due to the poor current gain of the BD131 and the 1k base resistor). I'd swap Q1 and R2 for a MOSFET like the MTP3055E (or something else cheap) - this will improve the overall efficiency as well (due to lower 'saturation' voltage and faster turn-on/off). At the very least, drop R2's value to ~100R (making sure you don't exceed the 555's output current capability for your supply voltage).
* Raise the capacitance values of the multiplier chain. 10n is a little low - 100n+ will make a significant difference. These type of multipliers are also very sensitive to leakage currents (i.e. small currents *drastically* reduce the output voltage due to their tiny current sourcing ability). At ~4kV you won't have to worry about corona effects, but sometimes potting these assemblies (into silicone/hot melt glue) can improve their operation significantly (and makes them easier to handle/mount).
* A high quality, HV output capacitor to ground might be an economical way to supplement lower-than ideal multiplier capacitances.
There's a start for you to begin tinkering. Fiddling with the oscillator frequency might change things too, but don't go too fast or you'll need to use fast (i.e. rapid reverse recovery) diodes in the multiplier chain.
Have fun (and don't forget to discharge the output capacitance before you fiddle with it! That's one you soon learn