Hello Sebastian,
I can see no particular problem with your KBI setup. PTA4 should interrupt on the rising edge. However, the other buttons should all be inactive for this to occur.
I assume you are actually using a QY4A device for your tests. If you were to use an earlier QY4 device, the KBIPR register is not present, and a positive edge cannot be selected. This was one of the enhancements for the A-suffix device.
Regards,
Mac