The following is an in-depth guide for the fourth area of Metroid II: Return of Samus.
First, fall down to the bottom of this shaft. Go to the left, drop down and go to the right. Go to the right and then you'll be able to fall down another small shaft. Head to the right until you reach a shaft, then go to the top of this shaft and go to the right when you get to it. Go to the right until you reach a wide open area. If you want, you can drop down to the bottom here to get a missile heal pool. Towards the right there's a big mound structure. You'll have to jump over this. After you do, drop down and go to the left. Shoot the blocks and go up to get the Space Jump. This allows you to infinitely spin jump in mid air if you get the timing just right. Go back down and to the right to get out. You need to get over the wall to the right so use your spider ball to stick to the walls and go over. Once you get over, you can Space Jump up high to get to a heal pool you can use. Go to the right and fall down to the very bottom og the shaft you find, then go right and go down another shaft to find and kill a Gamma Metroid. Go back up the shaft, to the left, then up the shaft again to get out. Go left a little bit and fall down the hole. In here you can find an Energy Tank. Go to the left and fall down the shaft until you find a right exit. In here you have to fight a Gamme metroid, but after you do that you can use your spider ball to climb up the sides and get a missile expansion at the top of the screen. If you want to get a Spazer, (Another Beam upgrade that replaces your Ice Beam that I personally wouldn't get,) you can go out to the left to a shaft, go to the bottom and then go to the left to get the Spazer. Go back to where you fought your last metroid and fall to the bottom of the screen. Go to the left and you'll fight an Gamma Metroid. If you want a Plasma, (ANOTHER beam upgrade that will replace you're Ice Beam, again, I wouldn't get it,) you can continue to the left and grab it. Next, go all the way to the left, grabbing the missile expansion along the way until you find another shaft. Go up, then go all the way to the left, defeating the Alpha Metroid and grabbing the Energy Tank. Go all the way back to the right bombing through the wall until you find the shaft from earlier. Go down the shaft. Go to the left until you find a room with a discolored ceiling. Shoot the ceiling and go all the way up the shaft and go left. You'll enter another shaft, go all the way up it and go right. Then go up using your spider ball to get out. Jump to the left, if you fall down the hole use the space jump or the spider ball to get out and on top of the left ledge. From here, jump to the left and you'll see a ledge and a door out, but you might miss it and fall into the hole, you'll have to jump out and get to the ledge to escape. Keep heading left until you reach a shaft, then go down a bit until you reach a right door, go through and go all the way to the save point. Fall down the shaft below until you find the first left door out, go through to fight a Gamma Metroid. Go to the right out of this room and go down the shaft just a bit until you get to the next right door. Go in and continue right to fight a Gamma Metroid. Go to the right to reach a shaft, go all the way to the top of this shaft and then go right to fight an Alpha Metroid. Use your spider ball to the top of this room to get out to the next room, where you need to go into your morph ball inside of the broken metroid to go to the next room, where you can fight the Gamma Metroid. In here there are a lot of things in the background that will destroy if you shoot them that makes this fight a lot harder. Use the morph and spider ball to get out of this room to the left, continue heading left until you find the shaft you were at earlier. Go down a bit until you find the first right door out of the shaft, go in it and continue right until you find another shaft. Drop down to the very bottom of this shaft and head through the left door. Then drop to the bottom of this shaft to fight a Gamma Metroid, then drop further and use your morph ball into this part to find a heal pool that will heal your HP. Keep climbing up and go to the left door at the top of this shaft, in this shaft go up to the first left door to fight a Gamma Metroid. Here go to the left and then go up when you can't go left anymore. Go all the way up until you reach a left door, go in it. go all the way left until you can't, then go up. Go all the way to the left of this room, then go up the winding path. Go all the way to the left and then go down to enter Phase 5.