Bela Bartok's Mikrokosmos is a large set of piano pieces which range, in order, from very easy to very hard. The pieces have lots of interesting harmonies and rhythms influenced by the music of Eastern Europe.
You can read an excellent in-depth, but not-too-long article about why Bartok wrote the pieces here.
Here's links to some tunes from Mikrokosmos you can play for a Grade 4 exam:
- Bartok - Mikrokosmos no. 87 - Variations
- Bartok - Mikrokosmos no. 90 - In Russian Style
- Bartok - Mikrokosmos no. 94 - Once Upon a Time
- Bartok - Mikrokosmos no. 96 - Jolts
- Bartok - Mikrokosmos no. 100 - In Folksong Style (just the first one in this video)
- Bartok - Mikrokosmos no. 108 - Wrestling
Picture from http://www.last.fm/music/B%C3%A9la+Bart%C3%B3k/+images/58911017
