The Fourth Colossus is a nice break from the last, much easier and faster to take down. This How To Defeat The Fourth Colossus In Shadow Of Colossus will walk you through the basic techniques to take down this beast to move on to the next Colossus.
This one is easier due to the environmental interaction required. You will notice in the area several small doors that lead underground, almost in a Hobbit style Shire approach. There are 4 doors total. Wait for the Colossus to get close and then enter through one of the doors. After a short time it will position itself atop the mound of that particular door. It will then bend its head slightly, lowering the two stone pillars that hang below its chin. You can jump out of the underground area onto these pillars and begin the climb.
There are two different weakpoint locations. The top of the head and the back of the neck. Both are very easy to reach and not too difficult to stay on. The neck has limited damage opportunities but the head can keep going right to the last.
Moving on, check out our guide for the 5th Colossus.