4 isn't the default for grave. Its default for coffin.
Sparky MB - 3 ball multiball. Complete the stand up pick targets, then hit in the snakes mouth for additional (20 seconds i believe) ball save, and adding a 4th ball.
Snake MB - 2 ball multiball. Same as above w/picks and ball save.
Grave yard/Cross MB - 2 ball multiball. Same as above w/picks and ball save.
Coffin MB - 4 ball multiball. Not possible to add a ball, however can add more time to ball save.
In game adjustments you should be able to adjust how hard each MB is to achieve. Meaning you can start the game with s, or sn, or snak etc spelled out for snake MB, etc. etc. How many balls per MB can't be adjusted.