In world 4-1 what big sticker do I place in the top section of the hill where the penguins slide downwards, I'm guessing this unlocks world 4-2, since I've looked everywhere else and only found an exit to 4-5 which I can't progress in because I need a scrap thats not in that level and something I thought led to an exit but just led to birdo on a rope swing giving me a goat?
First exit requires a fire-based battle sticker to be placed within the ice cream cone (in the very first segment of the stage).
Second exit requires a Radiator thing sticker, awarded to you by the "Sniffet or Whiffet?" host, after you accept and clear the special challenge. You'll want to do that game twice anyway for the door silhoutte in the final room (containing a thing in the resulting room) and also the newly appearing "HP+5" heart power-up.
Edit: While I'm at it, I'll mention that you should not enter 5-1 without a Vacuum thing sticker which is used to remove trash. If you've played through the previous Paper Mario titles, then pick up some trash before using the vacuum sticker. There's a lot of references and jokes in the trash that longtime followers will like.