The four pin header has:
Gnd Dclk Dio 3.3Apparently this is an SWD port. This is sort of a poor-man's JTAG. It can be used to program the chip, even replacing the boot loader. You could power the board from 3.3 volts using this header, but that would be kind of a backwards "hill-billy" approach.
Tom's Computer Info / tom@mmto.org