A guide to completing World 1-4 in Super Mario Bros 1 for the Nintendo Entertainment System.
Explore World 1-4
Go to the right and jump over the pit of fire. Farther to the right, there is a bar of spinning fireballs, also known as a fire bar. There is a ?-mark block above it that gives you a Mushroom or Fire Flower, but you have to be careful to jump on top of the ?-mark block if the fire bar is going to hit you.
When you're done there, jump over the fire pit on the right, then hold B to run as you go to the right. You will pass by three fire bars, and if you're running, they won't hit you. Then you'll reach a wider area with several more fire bars.
After you get past that set of fire bars, a flame will move across the screen toward you, so jump over it. To the right, there is a wider area, and there are six hidden blocks here that contain coins. Be careful of the flames that keep moving across the screen in this area. If you get on the floor and jump up and down as you go left and right, you should be able to find three hidden blocks. There are three more hidden blocks above the ones you revealed, but they are one square to the right of the revealed blocks. So get on top of each block you revealed, and jump up and slightly right to find the other three blocks.
When you're ready to move on, go to the right, and you will find Bowser, King of the Koopas and kidnapper of Princess Toadstool. You will have to dodge Bowser's fire breath throughout the battle. If you are Fire Mario, you can press B to hit Bowser with fireballs until he's defeated ( ), which allows you to go to the right side of the screen to get your reward. Otherwise, you can try to make a running jump over Bowser (or onto the platform above Bowser's head), or run beneath Bowser when he jumps up, but this runs the risk of getting stomped by Bowser, especially if you're tall. When you have gotten behind Bowser, jump on the axe on the right side of the screen to make the bridge collapse, sending Bowser into the flaming pit below.
Afterward, you will find not Princess Toadstool, but a Mushroom Retainer who says that their princess is in another castle! Then you will continue on to World 2-1.