Switched capacitor inverter
That is one ugly schematic.

But your main problem is that you have the body connections of M2 and M4 tied to ground. This means that no part of their channels (including the source and drain terminals) can go more than one diode drop below ground.

The body connection of an N-channel MOSFET must go to the most negative potential in the entire circuit in order to keep the implicit (channel isolation) diode reverse-biased at all times.

Also, you're going to have to re-think the gate drive for all of your devices. It would be simpler in many ways if you were to change M1 and M4 to P-channel MOSFETs.

A secondary issue is that the pump capacitor C0 isn't going to charge up to Vin; its voltage will be actually be Vsw minus the threshold voltage of M1, whatever that is. This is another reason to make M1 a P-channel device.