E4M1 Contains 5 secrets.

Secret1 : When you go up the stairs in the first room, stepping on the top stair will open a small panel on the opposite end of the room.
Contains : Doctor's bag

Secret2 : In the small library which contains the skull key, the middle bookshelf is a door. Open it.
Contains : Basic armour

Secret3 : In the passage before the bridge leading to the brain storage, there is a wall with a bloody hand print on. Push the wall on the right of the hand.
Contains : Voodoo doll

Secret4 : When you step on the top step before the brain storage room, a small panel will open in the passage below.
Contains : Limited invisibility

Secret5 : In the dining room, if you stand near the electricity and look towards the large door and look up, you'll see a golden face. Shoot it and a lift will be activated in the next corridor.
Contains : Sawed-off shotgun, a life leech, a life seed and a trapped soul

E4M2 Contains 3 secrets.

Secret1 : In the room with the spider queen, push the wall on the left of the room when you enter, it will slide away, revealing a secret compartment.
Contains : Doctor's bag

Secret2 : In the chamber with the spider queen, pull the pole around the central post. A panel will open some way through the eye door. It is in plain sight.
Contains : Life seed

Secret3 : In the outside chamber with the pool in the middle with the voodoo doll on top, there is a space in the central area underwater.
Contains : Super armour and a diving suit

E4M3 Contains 2 secrets.

Secret1 : In the tall area with all the cultists and the golden/brown bricks there is a pool in the center. Go for a swim.
Contains : Life seed

Secret2 : In the outside area near the end of the level with the water tower. Underwater, below the water tower, there is a space between some pillars.
Contains : Basic armour

E4M4 Contains 6 secrets.

Secret1 : When you exit the first wooden building, turn left and push against the forest. Enter the area and you'll find a masked wall area.
Contains : Voodoo doll

Secret2 : In the main dining room with the fireplace, when you move to exit the building and there's that small room on your right. Go around the left corner and push the wall. One is an elevator. Go up it.
Contains : Super armour, a box of shotgun shells, 2 tesla charges and 2 proximity mines

Secret3 : In the area with the bridge-like structure going over the water, with the napalm launcher in the middle. Go around the left side of the structure and you'll notice a bit of it near the water is a different colour. Push it and it will open to reveal an item.
Contains : Guns akimbo

Secret4 : Between the two doors, of which one leads to the moon key, push the wall and apanel will open.
Contains : Life seed

Secret5 : In the water, after you fall down the toilet, if you go right and use dynamite on the bars, you can enter a secret area.
Contains : 2 trapped souls

Secret6 : In the picture below, you can see a crack on the right of my crossair. This is along the wall to the right of the end of the seal of Tchernobog. Blow open the crack and you'll find the path to the secret level!

E4M5 Contains 5 secrets.

Secret1 : When you're leaping around over the lava and you're about to leap onto the brick surface for the first time (near the beginning) turn around, there'll be a nook.
Contains : Jumping boots

Secret2 : When you're walking around the ledge before you enter the skull door, there will be an object spinning on a small circular ledge to your right, in plain view. Jump to it.
Contains : Super armour

Secret3 : Before you enter the doors near the end, where all the skeletons are hanging, go to the far side of the ledge. There will be a lift. Just push your use key around the edges and the elevator will lower, revealing a secret ledge.
Contains : Life seed

Secret4 and 5 : These two are in fact linked. Where I pointed out secret one there is a small window. Use your jumping boots to jump in the window. There are a couple of ways of getting to these secrets. This is just one of them.
Contains : 2 gasoline cans, a box of shotgun shells and a doctor's bag

E4M6 Contains 1 secret.

When you cross the very first bridge, with all the cultists on, jump on the left stand with the gargoyle statue on and jump onto the high dirt ground on the side. There is a secret ledge.
Contains : Basic armour, a box of shotgun shells and a doctor's bag

E4M7 Contains 3 secrets.

Secret1 : In the one chamber there are two elevators moving up and down. Jump onto them and enter the chamber at the top.
Contains : Box of shotgun shells and a box of flares.

Secret2 : When you surface into the chamber where you push the skull key after you've gone underwater, search for a fairly visible skull switch above the one body of water. Shoot it and the pillar in the water opens.
Contains : Super armour

Secret3 : In the final room with the stomping objects, check the center post as you circle around it. There will be a small chamber.
Contains : Doctor's bag

E4M8 Contains 1 secret.

Secret1 : When you enter Tchernobog's throne room. Look at the wall above his throne. There is a small red button. Shoot the button and a panel to your right will open.
Contains : Guns akimbo

E4M9 Contains 5 secrets.

Secret1 : In K Marché (the mega store) there are two cash registers. Push the one on the left and a panel opens in the adjoining room.
Contains : Limited invisibility

Secret2 : In K Marché (the mega store) the one corner is wooden and conceals a crack. Use explosives on it to access a secret area.
Contains : Life seed

Secret3 : Before you jump through the grate at the end to access the ventilation shaft, look at the fan. There is an object behind it. Destroy the fan and jump into the shaft.
Contains : Super armour

Secret4 : In the ventilation shaft, before you access the storage chamber, look up, there will be a grate that gives access to a small upper chamber.
Contains : Doctor's bag

Secret5 : Here's a special one. All of that annoying music is coming from speakers (found on the ceiling). Destroy all five speakers and you'll find a surprise in K Marché.
Contains : Guns akimbo