That is from 2008. The variable bigmem controls it, in /usr/src/sys/arch/amd64/amd64/machdep.c and is disabled (set to zero) by default due to stability problems addressing more than 3GB of RAM. There have been many threads, one of them starts here:
http://marc.info/?l=openbsd-misc&m=124567074914895&w=2