It's finally time to run through Fabris' guards in Fourth, found here and here. These are the last set of single rapier guards in Fabris. After these, we'll touch on his lunge and girata from Fourth, and then I'll pick some other interesting Fabris thing to start in on!
The first guard in Fourth is, as has become a trend for Fabris, described as not very good. From here, Fabris recommends simply passing underneath your opponent's sword and straightening your arm as you do. As another option, you can turn your hand to Second. If you leave the tip of your sword in place, you'll create a "considerable motion" with your hand - you can see the angle of the forearm in the plate if you squint a little, and rolling into Second will have a pretty wide area of motion - and in doing so, you can use that motion to your advantage if your point isn't moving.
On to the second guard in this series, which Fabris states is much better, and "safer one than any other without comparison." That's a heck of a recommendation! Note that the lean of the upper body is much more pronounced than in the former, and that it is directed much more towards the opponent than off to the inside; this is owed in part to the hips being more properly positioned, as well. You can see the fencer on the left has much more squared off hips, and the one on the right not so much, but note the feet and how he's stepping into the guard somewhat. Additionally, the left arm is brought back in a position that we've seen in lunges before, which will keep the upper chest more narrowed and safe. Finally, the sword is in a much more extended posture, which will help encourage your opponent to stay at a distance from you, as well as allow you to perform cavazione and other blade movements in a much more efficient manner.
Fabris notes that this guard keeps you safe on the outside line as well as the inside. Your opponent can try to move your blade, but cavazione are so tight here that this is very risky. He could try to pass underneath, but your blade is so extended that he'd have to use a lunge, which takes a long tempo.
The third guard that Fabris shows us seems to give up a lot of those advantages. The upper body is much more squared off now, and there's a line to the upper chest inside the sword and off hand. The blade extension suffers due to the angle of the arm, as well. However, Fabris notes that the feet are spread a bit more, so that they're on either side of that line to the chest. Because of this, you can readily move either foot to displace your body safely away from the attack on whichever side you prefer.
Interestingly, Fabris notes that while it appears open to the outside (due to that arm position that we noted earlier), it's a fine invitation as you can pass with your left foot and strike your opponent underneath his sword, or even over it if you increase the angle with your arm. Fabris goes on to say that the more strongly your opponent tries to parry, the more easily he will be wounded. I can only see this working because of the angles of the swords involved - note that he doesn't suggest turning your hand into any other position, but simply maintaining the angle of the blades.
If you pass to the inside and turn your hand in Second, Fabris specifies that you need to be close enough to be able to get your head past your opponent's sword simply by leaning, or you will not be safe - but that you can also get your left hand onto your opponent's hilt.
Finally, the last guard is one that has more strength to the inside, because of the blade angle. Fabris is including this "to show a good way to operate against an opponent who is situated in an angled Second guard and find his sword." He notes that it's hard to oppose someone who really commits to a deep angle in Second, so you'll need to remember that a straight line will get you to your target easier than an angled one, to void, or wait for your opponent to close and then strike.
There's some odd things in the guards in Fourth, to be sure. Some of it continues to emphasize blade angle over hand position in determining which line a blade is strong on, which works against common wisdom in the SCA but is very much worth investigating. Additionally, the emphasis on extended guards remains strong, but we'll see that change somewhat when we eventually look at dagger guards.