There are four guards because there are four perspectives of the sword, the two flats and the two edges.
The first guard is the natural position of your hand when drawing the blade. (NOTE: SCA fighters really should be required to use sheaths in tournaments. Much of the art deals with those first few seconds when drawing the blade.)
The second guard is when you turn your hand downward some. Note that he isn’t using absolute terms like 12 o’clock and 3 o’clock.
The third position is in a natural position not turned either way. This is the only guard with a concrete position.
The fourth is turned inward some.
Fabris also mentions the bastard guards between each proper guard, but he doesn’t go into much detail. As far as he is concerned they simply have the characteristics of the adjacent, legitimate guards.
Fabris concludes this chapter by reiterating that each guard has many variations. (This took me a long time to learn. When I started out I assumed that the four guards were the only legitimate guards.)