Okay so once you collect 30,000 total Rupees, you unlock a special set of stages called the Hero Trials.
They are.
The Most.
Impossibly BULLCRAP stages in the entire Legend of Zelda series. Just... insane. For the first time in the series I am actually happy to see hearts come out of treasure chests.
I ended up getting a Game Over near the beginning of the third stage of the Gold Room, because it was such utter BS. One room has like... ten Moblins at once (an enemy that's already darn near impossible to beat without taking tons of hits) and there are these two fans that blow you against the wall, making it much more difficult to fight.
The prize for completing all three Hero's Trial rooms is the Hurricane Spin, which was difficult to obtain in the original Four Swords mainly because it required you to be the player to collect the most rupees during ten stages.
Single Player mode is much more difficult to win in general because of something I remember from my days playing Four Swords with my friends in High School. When every active player has full Hearts, the game goes into what is known as "Rupee Fever" where every rupee collected is worth double.
You get the Silver, Gold and Hero keys in each round of the normal stages by collecting 1000, 3000 or 5000 rupees respectively. I failed to get a gold key last night because I didn't make use of every opportunity to collect rupees. In the old days getting the Gold Keys was nothing, but now that I am forever alone...