Hello there,
In case you or other readers have not yet got into the F.I., here are some thoughts.
I agree with those who say that there are not "etudes" which would prepare for this piece. In a way it is its own etude. However, Brahms loved writing 3's against 2's and indeed 4's, so one of the easier Brahms pieces might be an idea.
The best technical practice preparation is to play every possible scale and every arpeggio many times. Recently I went to a new teacher who encouraged me to do this: and the results were dramatic!!
Then treat the 4 against 3 situation as an exercise in written theory, in order to decide exactly where each note falls. This involves subdividing each beat into "twelfths". Once you have worked this out, play a few fragments with both hands exactly thus, incredibly slowly. As if you are balancing, say, 4 MINIMS against 3 MINIMS each time. This will help to lock the patterns into their exact place, like cogs engaging into a piece of smooth machinery.
Finally, practice each hand separately, many, many times.
After this it will not be long before the urge to put the hands together becomes irresistable. And then you will be started ... if not finished.
The bits I find most difficult are: (i) the descending RH sequences around Bar 8 etc, and (ii) the free-form 7-tuplet at bar 60. Can anyone else help? I have been trying to play this piece for getting on for 50 years ..........