Chapter 3 “Hobgoblin” has 8 Hidden Spiders:
Hidden Spider 17 — You can find the 1st one on the ground at the top of a ledge on the second platform area that you come across from the beginning. You will need to web-jump up to this ledge to grab it.
Hidden Spider 18 — You can find the 2nd one in the building hallway you enter just after collecting #1. It is at the end of the hallway on the blue lighted circle on the wall above the door.
Hidden Spider 19 — You can find the 3rd one on the rooftop above the left doorway of the building. This will be when you have just learned how to use Accelerated Vision and when you fight a bunch of enemies including Hobgoblin once again.
Hidden Spider 20 — You can find the 4th one crawling on the side of the platform. This area is when you are passing through the billboard on both sides of you across the chasm. Make sure to grab this before entering the next room, or else you will have to replay again.
Hidden Spider 21 — You can find the 5th one above a doorway on another blue lighted circle. This is the room right after where you collected #4 off of the platform. Do NOT defeat the enemies in this room before you have collected the Hidden Spider. If you do so, you will have to replay the chapter all over again. This is because once all of the enemies have been defeated; a cutscene takes you out of the room and saves the game.
Hidden Spider 22 — You can find the 6th one above the chasm on the archway in the middle at the end. You can either web-jump up there or web-zip from arch to arch to collect it. This will be coming up shortly after collecting #5.
Hidden Spider 23 — You can find the 7th one on the ceiling in the hallway of the building you just entered after having collected Hidden Spider #6. You can either web-jump or crawl to grab it.
Hidden Spider 24 — You can find the 8th one above the chasm on the archway in the middle. You can easily obtain this just by web-jumping. But if you happen to run into problems, you will need to crawl on one of the walls and then onto the archway to grab it. This will be the next area after just collecting Hidden Spider #7.
This video shows how to find all 8 Hidden Spiders in level 3 “Hobgoblin”:
Chapter 4 “Electro” has 8 Hidden Spiders:
Hidden Spider 25 — You can find the 1st one inside the first room to the right as you enter the first tunnel at the beginning. Once you have entered the room, it will be above the doorway on the red bricked wall.
Hidden Spider 26 — You can find the 2nd one inside another room similar to the same room you found #1 in. This is at the end of the area where you save the civilians from the electrified water. You will need to web-jump upwards instead of going through the door.
Hidden Spider 27 — You can find the 3rd one at the end of the dam in the right corner on the left side of the power plant utility truck. This is just after encountering the green electric creatures with the lasers. Make sure to not head into the next tunnel before grabbing this hidden spider.
Hidden Spider 28 — You can find the 4th one on the ground behind a power plant utility truck just after crossing the first set of power lines over the river.
Hidden Spider 29 — You can find the 5th one just after another fight with electro in the generator field. It will be crawling in the rubble below the broken ledge in the tunnel.
Hidden Spider 30 — You can find the 6th one in the timed area with a ramp leading to the power station. It will be crawling on the wall at the back end of the pier, just below the corner of the ramp.
Hidden Spider 31 — You can find the 7th one in the same area as you found Hidden Spider #6. But instead this one will be at the top area, up the ramp, to the right of the entrance to the next building. It is behind the power generators crawling on the red bricked wall.
Hidden Spider 32 — You can find the 8th one in the building where you have to save 5 civilians, the same building you entered just after the area where you found Hidden Spider #6 and #7. It is in the far right corner crawling on the wall underneath the big number 2.
This video shows how to find all 8 Hidden Spiders in level 4 “Electro”: