After entering the second part of 4-2, you'll see whales. You have to travel through the whales.

You'll come to a potion after six pull-ups. Instead of throwing the potion there and getting six coins, keep the potion and travel with it it through half of the stage until you come to a pipe. Throw the potion by the pipe and go down it while in subspace. You'll warp to 6-1.