Changing documentation about ramdisk to use 4g instead of 2g

When following the instructions, I found that a 2g ramdisk wasn't large enough. I saw errors like:

"log: exiting because of error: write /tmp/vt/tmp/mysqlctl.aaijazi.aaijazi.log.INFO.20141030-110215.31092: no space left on device"

Bumping ramdisk size to 4g (who doesn't love powers of 2?) made tests pass successfully.

@enisoc