...
## default num
# Set the default entry to the entry number NUM. Numbering starts from 0, and
# the entry number 0 is the default if the command is not used.
#
# You can specify 'saved' instead of a number. In this case, the default entry
# is the entry saved with the command 'savedefault'.
# WARNING: If you are using dmraid do not use 'savedefault' or your
# array will desync and will not let you boot your system.
default 0
...
až otevřš ten soubor, najdi si tuhle sekci. Potom na řádku s default (u mne ten poslední) přepiš '0' na '4'...