2. Perfect legato. Listen for a perfectly smooth connection between the notes of each line. The hand should feel a steady balance as weight transfers from finger to finger. Keep that feeling in the legato part even when a break occurs in another.
To be fully prepared to play four independent lines at the same time, you need to be able to play each line flawlessly without even thinking about it. If you feel even the slightest challenge, practice until you can play with ease five or more perfect repetitions in a row, and memorize. Repeat that process over at least a three-day period.