You need to finish levels 1 to 3 first, then once you complete Level 3-5, there will be an option to go to Level 4 for 100 coins/1 HP, or finish the game
It's the original SK. You will need to finish the first 3 levels of the game first (1-1 to 3-5), and then once you finish level 3-5, there will be an option to continue to Level 4 or end the game for 100 coins/1 HP ;)