I've been playing piano and learning theory for almost 2 years now. The most difficult piece I've successfully learned was Aria in d minor (k32) by Scarlatti. A quick google search tells me that this piece is grade 4. But then when I look at other grade 4 pieces, one that really caught my attention was Prelude no. 2 in C minor from the Well Tempered Clavier by Bach. Is that a mistake? Because if you listen to both pieces you can hear a distinct difference in difficulty. But I just ordered a new piano off of Amazon so Im searching for new music to learn for when it arrives. And I'm just wondering if I should even attempt that prelude or will I only be disappointed when i realize I cant do it
Being able to play one grade 4 piece does not mean you will be able to play all of them. The C minor prelude looks and sounds harder than it actually is, in my opinion (as someone who has learned to play the piece). I would say the overall difficulty level between the two is comparable, although as Dr Mayhem commented, the pieces do very different things musically.
The Scarlatti has chords in the left hand and a melody line in the right hand, and both hands playing independent rhythms. Contrast with, the Bach C minor prelude has (for the majority of the piece) both hands playing a single line each, and in the same rhythm, also with the same cycles of repetition in each measure.
What makes the Bach difficult is the fingerings and the changes in fingering "settings" from measure to measure. You will probably want to play from an edition that has fingerings noted in the score; the process of figuring out fingerings can be a puzzle in itself (especially with Bach's work) that you may not be ready for.
In my experience learning the piece, it takes all of a couple seconds to learn to play a single measure of the Prelude in isolation -- what was difficult for me was learning to smoothly transition from one measure to the next and to keep both hands in sync while doing so.
And there's certainly no harm in sitting down with the piece for an hour and seeing for yourself how realistic it is for you. As I said in my very first point, you may need to go back to some different level 3 pieces that focus on these same skills before you are ready for this one.
I agree with all of what Mr. (Ms.?) Reilingh says except I would suggest that you don't practice "a measure at a time", rather practice in phrases (a musical sentence) by breaking the phrases into small manageable parts then combine these parts until you have the complete phrase. Granted this may result in "a measure at a time" but I believe this way of approaching your practice avoids the non-musical constriction measures sometimes impose.
When I was learning grade piano, some pieces required analytical practice, some I sight-read. But pieces don't define a grade, performances do. The way you play a scale for Grade 4 won't do for Grade 8 - and it's little to do with adding a further octave or two.