I express gratitude for your help.

The passage with fire-balls has two perpendicular branches. Nearby branch has the red door. and the far (south) branch  has blue door. You run through  the blue door and search a door (with long room and three robots) at east end of this branch.. This room has red key (may be, you must jump over a pit in this room, may be, I am mistaken about the pit),  .

I'm actually feeling like an idiot currently because I keep asking questions and, with only one exception thus far, figuring out the answers myself via trial and error.
I took the fourth load slot for a spin, explored that area near the red door with full health and fairly good ammo...and found exactly what was being talked about. Got the red key, got all the other crud, found my way to L5 but with about 9 health, and will take L5 for a spin (a true one) when I actually go through and beat L4 all the way through by myself).
Sorry for asking all these stupid questions, by the way.

I will have the readme.txt document proofed within the next fifteen hours. Where do you want it posted/sent?

Missed the medkit that time. This time, I got it, but L5 is a real tough one! Stupid belts!
You've got to have some mighty impeccable timing, and the enemies seem tougher, but that could just be me. I am dying for a toilet or a medkit at this point.

Well. In the second story, I seem to enter a door and then hear a click and bang! I'm dead. I know it is for that grenades, but, well, how to know which door I am not allowed to open? You know, I turn around and in a while loose the sense of direction and things... You know what I mean.

Wanna see an absolute kick in the teeth?
I have a save file on L6, at the place I mentioned in my last post. The one time I manage to get through the doorway thing, I save just after clearing it. Well...I got killed because I opened a door and got a rocket in the face. I reloaded, and, get this: there's high voltage directly in front of me, my character can't move, and it says I've failed. I've gotten lucky twice and it asks me to input the correct code, but the joke's on me because I didn't write it down. Besides, when I actually saved I wasn't touching the electricity so I don't see why I'm now stuck on it. Definite glitch there.

Levels 1-3 are a snap, L5 is tricky at some points but not bad...it's L4 I really don't like much.

Slight spoilers ahead:
In the room with many mines, you have two worthwhile choices:
1. If you have a hand grenade, chuck it and RUN!!! Right back out the way you came, and you're still likely going to take a little damage.
2. Save just inside the door, preferably after killing the soldier in here cuz he'll annoy you otherwise. Then tiptoe your way around.

If you already have the trapped red key, however, this room is utterly useless to you. Go to the end (because it means you've likely already discovered the elevator).

Slight
spoilers

Level 4 has to be one of my favorite levels; I love that timed dor maze (following the medkit etc) and Level 3's awesome design.  There's a ton of stuff that you might not notice unless your careful; some, like the switch in level 4 (which I figured out after falling down that huge pit like five times) look like they're the way out of a level.  We definitely need more accessible games with this kind of challenge.  I'm on level 5 trying to navigate the afformentioned maze of fireballs; I got through two passages.  This game is really awesome - I can't stress it enough (hey, positive feedback increases the chance of more stuff like this coming out).

Slight
spoilers

Aaron: if you are facing the switch, the door you're seeking is to your left. You need to step back a couple paces, strafe left, then move directly forward. The Duck command is very much your friend, else you're going to get pulverized.

To the other poster navigating the fireball maze, I want to offer one tiny tidbit that may relieve frustration, but it is
slightly
spoiler
tastic:
Think of it as an accordion...strafe right, go up a couple steps, then strafe left, etc. ... though it still isn't going to be easy. Nice rewards when you get to the end though. And after that, well...heh, you're on your own.

The grenades should be sounding like normal items. There are many of them, so if you're thinking to yourself, "Jackpot, baby!" like I did...nope nope, those are all geared to blow you to bits.

In response to how far away from the l6 switch should you be:
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Stand about 6 meeters from the switch and shoot (I use the rifle).  Turn around, take a few steps and jump the pit.  Quicklly go through the opening (it will close up in about 10 seconds or so, so if you don't make it shoot the switch from 6 m again).  You better have memorized or written down the code.

I never got stuck in that way you described.  I always load from the first hall of level 6.

Hi.
This is sure  a stupid question. I'm not so very good at this game, but in level 1, Got the axe and rifle, and defeated the robot. I'm completely stuck, where shall I go? All I can see is acid and walls, and a switch. So can anybody help?

You probably have lost the sense of direction. I myself, to prevent this, navigate using ctrl+left or ctrl+right instead of turning, and always keep one direction (usually north) to not get lost. If you look around, after you press that switch, there are doors that you go32:"a9a1b74d078a7f2d0e5905d852c1c065";}