The watts-to-HP conversion has nothing to do with voltage.
4kW = 5.36HP
That's not to say you could START a 5HP motor with a 4kW converter, that is an issue having to do with the overload capacity of the converter.
Back to the OP;
You can buy or build a rotary phase converter for a lot less than a VFD capable of converting for a 5HP 230V motor, because to do it with a VFD, you would actually need to buy a 10HP drive*.
You can see plans to make your own, including formulas for sizing components, on this
forum for machinists. But I agree with the comments that it would likely be less expensive and more reliable to just swap out the motor with a single phase one, especially when you factor in the value of a 5HP 230V 3 phase motor on eBay.
* There is possibly one manufacturer who makes a 5HP VFD capable of 1 phase input without de-rating.