damn, i love submachine.
spoiler warning! where the 21 secrets for submachine 4 are, and what theyre used for!
In the lab:
1) on the ancient chair all the way on the right of the attic
2) first floor down from the attic
3) underneath the computer table where u talk to Mr.M
4) behind the chair next to the stair way all the way at the bottom
5) right next to the gas valve standing next to the burner.
6) behind the pipe on the left side of the bathroom
In the ancient section:
7) in the rubble underneath the "+" sign
8) underneath the statue of the horse thing? break the crumbling box with the hammer
9) the eye of the dog where you put the 4 triangle pieces
In the looping trap:
10) in the corner of the room with coordinate (0,-1)
In the basement section:
11) on the socket in the room with the coil charger
12) behind the laserbeam machine on the right side
In the room coordinate (5,5,1)
13) on the right side of the portal machine
In the light house digout:
14) on top of a pile of dirt under where the pipe turns in an "L" shape
15) one room to the left of the portal in a square below the 4 levatating squares. to get this one, you need to go through the submachine and find the four bells, ring them, and make them levitate the squares.
In the tomb trap:
there are none
In the 2nd tomb trap:
16) all the way in the corner on the left side
In the ship:
17) next to the lantern in the same room as the portal
18) one room right of the portal. you need to unscrew the 4 screws on the square on the wall.
19) behind the lantern on the top floor, where u stick the fan in.
(NOTE: by now you can go back to the lighthouse digout to get that 15th secret)
In the ending room:
20) in the corner on the room on the right, where the portal machine is. it appears white here.
In the room with the coordinates (7,2,8)
21) in the circle in the upper left hand corner
you put the secrets in the tube in the right on the right. hope ive helped, and hope youve had fun!