How do I change the default LED pin for displaying uploading I made a board for my project based on Arduino Pro Mini. The problem is, that I had to put the LED on pin 4 instead of pin 13.

What setting do I have to change to make pin 4 the default one? Or do I have to write my own Board Library?
The blinking while uploading makes the bootloader. Arduino Mini uses Optiboot. The source codes are on GitHub. You would need to change the pin in pin_defs.h and make the bootloader. Then flash it to the 328p with a programmer over ICSP.

Details what to change are in Greenonline's answer.