Well, it depends on the pieces you are learning, overall I would say 4 pieces at once seems fine, as long as you are able to practice theory and technique as well. Something I find helpful when sightreading, or even listening to a piece is to look at a copy of the sheet music and try to follow along. Then, attempt the piece for yourself. This has helped me! Also, take a few measures of the right hand, get that down, and then the measures of the left hand. It really moves you along!
All the best,
Natalie