How do I connect 4 electrical wires to a 3-phase cooktop?
Since you are in Denmark, your mains are 400 V phase-phase, and 230 V phase to neutral. The white and brown wires are two of the three phases, and the yellow/green is your ground. You don't have a wire for the third phase available.

The standard IEC color code says that blue is neutral, brown is L1, grey (what you call white) is L3.

Your stove, since it doesn't have the screw for L3, presumably only needs two phases of power, neutral, and ground. This is what you have available.

You should connect yellow/green to PE and blue to N. I'd connect grey to (1) and brown to (2), preserving the ordering of the phases. The configuration of these two "hot" wires likely does not matter, as the cooktop's loads are likely all connected phase->neutral instead of phase->phase.