But before I apply more power I would like to know from Velleman if the PCB heat bed can handle 24V.
Well: What could happen?
The bed is a resistor, designed to produce heat. If driven with a larger voltage, it gets hotter.
Given 15V/2.4A it has a resistance of 6.25 Ω.
When 24V are applied, the resistance results in a current of 3.84A, which in turn results in roughly 92W, about 150% more than the original design.
Since there's a PTC sitting right in the middle of the business, any damage to the hotbed could only happen if the power input to the hotbet would heat it so fast that the controller would be unabled to turn off the power fast enough to prevent overheating.
I severely doubt that. To overheat an area of 400 cm² within a few seconds, you'd need much more power.
It might be advisable to replace the heatbed wiring from four parallel ribbon cable conductors to something more substantial, but otherwise only the ribbon cable would get warm.
With just 4A, not that very much.