Location of All 4 Hidden Blueprints in The Forest
In addition to all of the original recipes in the Survival Book you can find 4 additional pages of Blueprints that give you even more items to create in The Forest. Each of these Blueprints is hidden inside of an underwater cave which is found off of the coast of the main island.
Use my screenshot above if you'd like to see the exact location of every hidden Blueprint in the game. To reach all of these locations I strongly recommend you build either a House Boat or a Small Raft as it'll be much safer and faster to travel in one of those to the location then swim down to the cave from there.
You'll also need the rebreather in order to enter all of these caves since they have underwater entrances. A tip for quickly finding each of these locations, they'll be marked with a floating piece of wood that has two yellow ropes attached to it. I've included a screenshot of this piece of wood below so you can see what you're looking for.
Tip: If you are having trouble seeing inside of these caves (or in the dark) change the Color Grading settings under Graphics in the menu from whatever it is now to M131 - it will typically significantly brighten up your game at night. It's a night and day difference!
Blueprint #1 - Cross, Church and Coffin
The first Blueprint for you to find is in a small cave located below a shipwreck which can be found in the southwest corner of the map (bottom left). I find the best way to locate this one is use the island as a marker, there is a southern peninsula on the island and the floating log you'll want to look for is directly south of it.
Blueprints #2 & #3 - Glider & Tower
Due north of the previous cave is where you'll find this one. Easiest way to get here is to ride your Small Raft or House Boat all the way up the coast from the first cave. When you pass the shipping containers try to spot the floating log to the south of the small dot on the southern island.
Another way of looking at it is, along the coast north of the shipping containers there is a lump of land sticking out into the ocean - the log lines up pretty well with this lump of land if you started there and went directly west. Use my map above for better directions.
Schematic #4 - Log Track
Last but not least you have the underwater cave located in the southeastern part of the map. You can find the entrance to this cave just off the southeastern coastline inbetween the coast and a strip of islands. It's easy enough to spot from the coastline and you can swim to the floating log that marks the cave from the shore.
After you make it inside of the cave you'll have to swim down again once you're in that cave to reach the Blueprint along with a few more Crossbow Bolts.