Locating the Zone entrance[edit]
Now that you have one important item, the Air Helmet, you can use it to collect another item that will be vital to your passage through the remaining zones, the Jump Shoes. However, you can explore the house from one visible end to the other, and never find the zone you're looking for. To find it, you actually need to locate a way to access the attic. It may be difficult to find purely by luck, but the access your looking for is hidden away in Room J. That is where you should start your search.
Room J[edit]
You may have visited this room once before, just prior to visiting Zone 2, but you'll absolutely need to visit it if you have any hopes of finding Zone 4. Just like the items in the room, everything you're looking for can be found along the North wall, including a spot on the upper left corner of the wall which you can hit to reveal a ladder leading up to the attic. After you discover it, point to it and "GO" up the ladder to Room N.
Room N[edit]
Taking the trip up to the attic will deposit you in a new room, facing South, where you can find machine gun ammo in the little cupboard. But if you exit out the door to the attic hallway, you won't find much of interest. In fact, you won't find anything at all so restrict your search to Room B. On the West wall, you will find a valuable potion, and on the East wall, you will find a door that will lead you to hidden room b.
Hidden room b[edit]
You will arrive in hidden room b facing East. In this direction, you can find yet another potion, which will be useful since you should not enter Zone 4 unless you are at, or very close to, full health. Then face north and open the left door to find more grenades. Finally, open the door on the right and "GO" inside to access Zone 4.
Into Zone 4[edit]
Zone 4 is entirely underwater, and you will clearly not survive if you do not possess the Air Helmet. Moving through the water can take some getting used to. Bolstered by your own buoyancy, you can jump rather high through the water, and you tend to sink very slowly. You should have a good understanding of you move through the water in order to minimize your chances of colliding with the fish.
Initially, you will have to take the high path to get over the dead end on the floor, but after that, you should take the lower path, as it is the upper path that dead ends next. Once you are on this lower path, there are no more decisions to make, as the remainder of the level consists of a single tunnel.
The fish in Zone 4 are fairly menacing, as they move back and forth quite randomly and are hard to predict. While you can use your knife to remove them from the area, it might be wiser to attack them from a distance with your gun if you can afford the bullets. You won't face any other enemy on your way to the boss, but they will present quite a challenge on your way, especially when they are located above you in a narrow space.
Boss[edit]
The boss of the fourth zone actually has two different forms. It starts out like a giant sea turtle that very slowly advances towards you, just as the feable boss of the first zone does. All you need to do to defeat it is stay safely to the left and shoot it a couple of times with your gun. After a few shots, it will appear to be destroyed, sending five pieces flying into the water. The pieces, however, soon develop into smaller, faster, and more dangerous versions of the original turtle.
The smaller versions move through the water much more quickly and less predictably than the original turtle did. It can be challenging to attack a specific target. The best strategy is to remain on one side of the screen, and jump up and down while spraying the water with machine gun fire. Each bullet may hit one of the small turtles as you fire. Eventually, you will whittle the turtles down until you eliminate the final one. Then you can collect the fourth laser gun piece, as well as the vital item you came for, the Jump Shoes. Your journey back to the zone entrance should be very similar to your journey to the boss.