Shadow of the Colossus/Phaedra - Colossus 4
 Reaching Phaedra
Phaedra can be reached by going east of the Shrine of Worship. You will reach two paths - take the south one. Follow the path through to the plant-filled valley.
Ride Agro down the path into the valley. Though he will be available for this fight, he is essentially useless. You'll notice a set of four openings arranged in a square when first entering this "arena." These will be key in defeating this colossus. Moving a little further down will start the cutscene that shows the fourth colossus awakening and ultimately begin the battle. Say hello to Phaedra, the War Horse.
 Defeating Phaedra
Phaedra appears to move slowly and elegantly, but he can cover a deceptively large amount of distance in a few strides. Immediately turn around and sprint for the caves. Jump down into one and wait near the entrance. Unlike the others so far, Phaedra requires an element of stealth (or simply cowardice) to defeat. You'll know that he's above you when the ground really starts to tremor (it may even knock the hero off his feet) and dust falls from the ceiling. Phaedra will spend some time stabbing the bunker with it's sharp, pointed feet, so stay inside to avoid being impaled. Once you're sure he is above you and has stopped stamping, there are two options for how to proceed. The first is to simply wait in that spot until Phaedra lowers its head. When it does so, you'll see a pair of (what look like) long dangling earrings, which you can grab onto if you run and jump. From here, simply jump-climb your way up to the top of its head, where the weakpoint lies. As usual, charge-stab until he falls. The problem with this strategy is that climbing up Phaedra's earrings wastes a lot of stamina, so the probability of falling off his head increases. He tries to knock you off a LOT, so beginners should probably try the second strategy.
This other technique involves coming up behind Phaedra while he's thrashing about above the cave you originally jumped in. Run around in the cave and come out of one of the other holes. Stay hidden and try to get the camera so you can look over the hill where the hole is at the Colossus. If you've done this right, he won't notice you. Wait until your opponent clambers over and then squats down to try and peer into the cave.
Next, run towards his rear and you will notice that his tail (for lack of a better word) is fairly close to the ground, thanks in part to the elevation. Jump and climb up this onto his back. It can be quite hard to judge this jump, for some reason, but thankfully Phaedra stays like this for a long time. On his back, climb up his fur, and rest if you need to. Once you're ready, go to the base of Phaedra's neck and repeatedly stab the mini-weakpoint (similar to the point on the first colossus' ankle). He'll lower his neck and you can climb over the armour and reach his head. You can defeat him with a few fully-charged stabs.
 Hard Mode Strategy
You'll still need to get on Phaedra's head. Once you have taken off a little more than half of your opponent's health, the weakpoint will disappear. On the shoulder areas on both of his front legs are two magical weakpoints for you to take care of. Charge-stab both of these to take out the remaining portion of Phaedra's life; you should probably rest between each shoulder attempt. Once they're both gone, you win.