6000W cooktop circuit

If I were in the same situation, I'd hire a licensed electrician. It's safer and protects your property as well as your safety. Why take such a gamble?

Sorry Hairy, no offense, I realize how complicated English seems, but it's not clear to what extent the cooktop increases the unit's total consumption. I'd guess then to about 8400 w

Some of us unfamiliar with the term "sub circuit" are uncertain where exactly it's connected. Incidentally you haven't specified the voltage in either case

Very unsure because of above uncertainties, the source of power, its voltage; but now I'm assuming 220 v with the two breakers coming off single bus capable of 52A min

Not sure Hairy what you mean by "both appliances." Clearly one is the existing oven but then the other must be the cooktop. If by the "existing" circuit you're referring to the 32A line then no of course not, unless your inference is 440v, in which case ignore this entire posting)

At 220 v a 32A breaker will support about 7 kw; pushing your luck a little but my guess it's okay

Of course at 220 v, 20A is 4400 w which should easily handle the oven, although some might say that's overprotection

However there are still issues pending. For instance I'm wondering if you had intended one breaker to be downstream of the other, an absolute no-no as I understand it

As for wire size, etc, of course Google

You're welcome though Hairy

Forgive pun

The thing to remember in all cases is the breaker protects the wire from overload not the appliance.
Cookers are allowed diversity since they are never totally swiched on, but once again its the cable that matters if most cookers were at full load the breaker would go.

The only part I'm concerned is the size of the wiring. You must try to read the printed letters and numbers on the cable to check its size. And from here, to check how much load can handle.

I never heard of a 4mm wire (diameter of the copper/metal conductor?).

I guess that one can't lean of such a diameter size because a smaller diameter of copper cable can hold more amperage than a similar diameter aluminum cable.

You better check the size of the cable in better technical terms, unless such is how it is labeled at your place... still, you also check what kind of metal is the conductor for the appliances.